App.xaml.cs

using MongoDBApp.Services;
using MongoDBApp.ViewModels;
using MongoDBApp.Views;
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Threading.Tasks;
using System.Windows;

namespace MongoDBApp
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

                     
            var app = new ApplicationView();
            var appVM = new ApplicationViewModel();
            app.DataContext = appVM;
            app.Show();
            
                         
        }

    }
}

ApplicationView.xaml

<Window x:Class="MongoDBApp.Views.ApplicationView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:views="clr-namespace:MongoDBApp.Views"
        xmlns:vm="clr-namespace:MongoDBApp.ViewModels"
        Title="{Binding UserName}"
        Width="800"
        Height="500"
        WindowStartupLocation="CenterScreen">

    <Window.Resources>
        <DataTemplate DataType="{x:Type vm:CustomerDetailsViewModel}">
            <views:CustomerDetailsView />
        </DataTemplate>
        <DataTemplate DataType="{x:Type vm:CustomerOrdersViewModel}">
            <views:CustomerOrdersView />
        </DataTemplate>
        <DataTemplate DataType="{x:Type vm:OrderStatisticsViewModel}">
            <views:OrderStatisticsView />
        </DataTemplate>
    </Window.Resources>

    <Window.DataContext>
        <vm:ApplicationViewModel />
    </Window.DataContext>


    <TabControl ItemsSource="{Binding PageViewModels}"
                SelectedItem="{Binding CurrentPageViewModel}"
                TabStripPlacement="Top">
        <TabControl.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Name}" />
            </DataTemplate>
        </TabControl.ItemTemplate>
        <TabControl.ItemContainerStyle>
            <Style TargetType="{x:Type TabItem}">
                <Setter Property="IsEnabled" Value="{Binding IsEnabled}" />
            </Style>
        </TabControl.ItemContainerStyle>
    </TabControl>
</Window>

ApplicationViewModel.cs

using MongoDBApp.Common;
using MongoDBApp.DAL;
using MongoDBApp.Models;
using MongoDBApp.Services;
using MongoDBApp.Utility;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;

namespace MongoDBApp.ViewModels
{
    public class ApplicationViewModel : ObservableObject
    {
        #region Fields

        private ICommand _changePageCommand;
        private IPageViewModel _currentPageViewModel;
        private List<IPageViewModel> _pageViewModels;
        private string _authenticatedUserName;
        private static IDataService<CustomerModel> customerDataService = new CustomerDataService(CustomerRepository.Instance);
        private static IDataService<Country> countryDataService = new CountryDataService(CountryRepository.Instance);
        private static IDataService<OrderModel> orderDataService = new OrderDataService(OrderRepository.Instance);
        private static IDialogService dialogService = new DialogService();
        private static IAuthenticationService authoService = new AuthenticationService();
        




        #endregion

        /// <summary>
        /// Initializes a new instance of the <see cref="ApplicationViewModel"/> class.
        /// </summary>
        public ApplicationViewModel()
        {
            // Add available pages
            PageViewModels.Add(new CustomerDetailsViewModel(customerDataService, countryDataService, dialogService));
            PageViewModels.Add(new CustomerOrdersViewModel(orderDataService));
            PageViewModels.Add(new OrderStatisticsViewModel());
            PageViewModels.Add(new LoginViewModel(dialogService, authoService));


            // Set starting page
            CurrentPageViewModel = PageViewModels[0];

            Messenger.Default.Register<string>(this, OnLoggedInMessageReceived);
        }

        #region Properties / Commands

        /// <summary>
        /// Gets the change page command.
        /// </summary>
        /// <value>
        /// The change page command.
        /// </value>
        public ICommand ChangePageCommand
        {
            get
            {
                if (_changePageCommand == null)
                {
                    _changePageCommand = new RelayCommand(
                        p => ChangeViewModel((IPageViewModel)p),
                        p => p is IPageViewModel);
                }

                return _changePageCommand;
            }
        }

        /// <summary>
        /// Gets the page view models.
        /// </summary>
        /// <value>
        /// The page view models.
        /// </value>
        public List<IPageViewModel> PageViewModels
        {
            get
            {
                if (_pageViewModels == null)
                    _pageViewModels = new List<IPageViewModel>();

                return _pageViewModels;
            }
        }

        /// <summary>
        /// Gets or sets the current page view model.
        /// </summary>
        /// <value>
        /// The current page view model.
        /// </value>
        public IPageViewModel CurrentPageViewModel
        {
            get
            {
                return _currentPageViewModel;
            }
            set
            {
                if (_currentPageViewModel != value)
                {
                    _currentPageViewModel = value;
                    OnPropertyChanged("CurrentPageViewModel");
                }
            }
        }



        /// <summary>
        /// Gets or sets the name of the user.
        /// </summary>
        /// <value>
        /// The name of the user.
        /// </value>
        public string AuthenticatedUserName
        {
            get
            {
                return _authenticatedUserName;
            }
            set
            {
                if (_authenticatedUserName != value)
                {
                    _authenticatedUserName = value;
                    OnPropertyChanged("AuthenticatedUserName");
                }
            }
        }

        #endregion

        #region Methods

        private void OnLoggedInMessageReceived(string username)
        {
            AuthenticatedUserName = username;
        }
       
        private void ChangeViewModel(IPageViewModel viewModel)
        {
            if (!PageViewModels.Contains(viewModel))
                PageViewModels.Add(viewModel);

            CurrentPageViewModel = PageViewModels
                .FirstOrDefault(vm => vm == viewModel);
        }

        #endregion
    }
}

LoginView.xaml

<Window x:Class="MongoDBApp.Views.LoginView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:b="clr-namespace:MongoDBApp.Behaviors"
        xmlns:btv="clr-namespace:MongoDBApp.Converters"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        Width="300"
        Height="300"
        ResizeMode="NoResize"
        d:DesignHeight="300"
        d:DesignWidth="450"
        mc:Ignorable="d">


    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="2*" />
            <RowDefinition Height="1*" />
            <RowDefinition Height="1*" />
            <RowDefinition Height="1*" />
            <RowDefinition Height="1*" />
            <RowDefinition Height="1*" />
            <RowDefinition Height="1*" />
            <RowDefinition Height="1*" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width=".1*" />
            <ColumnDefinition Width="1*" />
            <ColumnDefinition Width="2.5*" />
            <ColumnDefinition Width="2*" />
            <ColumnDefinition Width="2*" />
            <ColumnDefinition Width="1*" />
        </Grid.ColumnDefinitions>



        <Label Name="label1"
               Grid.Row="2"
               Grid.Column="2"
               Grid.ColumnSpan="2"
               Height="28"
               HorizontalAlignment="Left"
               VerticalAlignment="Center"
               Content="Username:" />
        <TextBox Name="UserTextBox"
                 Grid.Row="2"
                 Grid.Column="3"
                 Grid.ColumnSpan="3"
                 Width="150"
                 Height="23"
                 Margin="0,0,0,2"
                 HorizontalAlignment="Left"
                 VerticalAlignment="Bottom"
                 Text="{Binding UserName}" />
        <Label Name="label2"
               Grid.Row="3"
               Grid.Column="2"
               Grid.ColumnSpan="2"
               Height="28"
               HorizontalAlignment="Left"
               VerticalAlignment="Center"
               Content="Password:" />
        <PasswordBox Name="PasswordTextBox"
                     Grid.Row="3"
                     Grid.Column="3"
                     Grid.ColumnSpan="3"
                     Width="150"
                     Height="23"
                     HorizontalAlignment="Left"
                     VerticalAlignment="Center">
            <i:Interaction.Behaviors>
                <b:PasswordBoxBindingBehavior Password="{Binding Password}" />
            </i:Interaction.Behaviors>
        </PasswordBox>

        <Button Name="LoginBtn"
                Grid.Row="4"
                Grid.Column="3"
                Grid.ColumnSpan="2"
                Width="80"
                Height="22"
                HorizontalAlignment="Left"
                Command="{Binding LoginCommand}"
                Content="Login" />

    </Grid>
</Window>

LoginViewModel.cs

using Auth0.Windows;
using MongoDBApp.Common;
using MongoDBApp.Messages;
using MongoDBApp.Services;
using MongoDBApp.Utility;
using PropertyChanged;
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Security;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Forms;
using System.Windows.Input;
using System.Windows.Interop;
using System.Windows;

namespace MongoDBApp.ViewModels
{
    [ImplementPropertyChanged]
    class LoginViewModel : IPageViewModel
    {

        public string Name { get; set; }
        public bool IsEnabled { get; set; }


        public ICommand LoginCommand { get; set; }
        private IDialogService _dialogService;
        private IAuthenticationService _authService;



        public LoginViewModel(IDialogService dialogService, IAuthenticationService authService)
        {
            this._dialogService = dialogService;
            this._authService = authService;
            LoadCommands();
        }


        public string UserName { get; set; }
        public SecureString Password { get; set; }


        private void LoadCommands()
        {
            LoginCommand = new RelayCommand(OnLogin);
        }

            
        private async void OnLogin(object obj)
        {

            var result = await _authService.LoginAsync(UserName, Password);

            if (result)
            {
                Messenger.Default.Send<string>(UserName);
                System.Windows.MessageBox.Show("You are logged in");
               
            }
            else
            {
                System.Windows.MessageBox.Show("Unknown username or password.");
            }
        }


    }
}