App.xaml.cs

public partial class App : Application
    {

        private static IDialogService dialogService = new DialogService();
        private static IAuthenticationService authoService = new AuthenticationService();
        private LoginViewModel loginViewModel { get; set; }


        protected override void OnStartup(StartupEventArgs e)
        {
            base.OnStartup(e);

            // Login
            var login = new LoginView();
            var loginVM = new LoginViewModel(dialogService, authoService);
            login.DataContext = loginVM;
            login.ShowDialog();

            if (!login.DialogResult.GetValueOrDefault())
            {
                Environment.Exit(0);
            }

            // If login is successful, show main application
            var app = new ApplicationView();
            var appVM = new ApplicationViewModel();
            app.DataContext = appVM;
            app.Show();
            
                         
        }

    }

LoginViewModel.cs

[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.");
            }
        }


    }