RegisterPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:converters="clr-namespace:App.Core.Converters"             
             x:Class="App.Core.Pages.Account.RegisterPage"
             Title="{ Binding PageTitle }">
    
    <ContentPage.Resources>
        <ResourceDictionary>
            <converters:InverseBoolConverter x:Key="notConverter" />
        </ResourceDictionary>
    </ContentPage.Resources>
    
    <ContentPage.Padding>
        <OnPlatform x:TypeArguments="Thickness">
            <On Platform="iOS" Value="0,20,0,0" />
        </OnPlatform>
    </ContentPage.Padding>
    
    <Grid
        RowSpacing="0"
        ColumnSpacing="0">

        <Grid.RowDefinitions>
            <RowDefinition Height="2*" />
            <RowDefinition Height="6*" />
            <RowDefinition Height="1*" />
        </Grid.RowDefinitions>

        <BoxView 
            Grid.Row="0"
            BackgroundColor="#00CCD6" />

        <Image
            Grid.Row="0"
            Source="brand_logo.png"
            VerticalOptions="Center"
            HorizontalOptions="Start"
            HeightRequest="75"
            WidthRequest="75"
            Margin="20,0,0,0"/>

        <Label 
            Grid.Row="0"
            Text="REGISTER"
            FontSize="Large"
            TextColor="White"
            HorizontalOptions="End"
            VerticalOptions="Center"
            Margin="0,0,20,0"/>

        <ScrollView
            Grid.Row="1">
            <StackLayout                           
                VerticalOptions="Fill"
                HorizontalOptions="Center"
                Spacing="15"
                Padding="0,15,0,0">

                <StackLayout HorizontalOptions="Center" Spacing="5">
                    <Label 
                        Text="Full Name:" />
                    <Entry 
                        Text="{ Binding FullName }"
                        WidthRequest="300" />
                </StackLayout>
            
                <StackLayout HorizontalOptions="Center" Spacing="5">
                    <Label 
                        Text="Email:" />
                    <Entry 
                        Text="{ Binding Email }" 
                        Keyboard="Email"
                        WidthRequest="300" />
                </StackLayout>

                <StackLayout HorizontalOptions="Center" Spacing="5">
                    <Label 
                        Text="Confirm Email:" />
                    <Entry 
                        Text="{ Binding ConfirmEmail }" 
                        Keyboard="Email"
                        WidthRequest="300" />
                    <Label 
                        Text="Email addresses do not match" 
                        IsVisible="{ Binding IsEmailMatching, Converter={StaticResource notConverter} }"
                        FontSize="Small" 
                        TextColor="Red" />
                </StackLayout>

                <StackLayout HorizontalOptions="Center" Spacing="5">
                    <Label 
                        Text="Password:" />
                    <Entry 
                        Text="{ Binding Password }"
                        WidthRequest="300"
                        IsPassword="True" />
                </StackLayout>
            </StackLayout>
        </ScrollView>

        
        <!-- FOOTER row -->
        <StackLayout
            Grid.Row="2"
            Orientation="Horizontal"
            BackgroundColor="#07616c">

            <Label 
                Text="Already have an account?"
                TextColor="White"
                HorizontalOptions="Start"
                VerticalOptions="Center"
                Margin="10,0,0,0">
                <Label.GestureRecognizers>
                    <TapGestureRecognizer NumberOfTapsRequired="1" Command="{ Binding BackToLoginCommand }" />
                </Label.GestureRecognizers>
            </Label>

            <Button
                Text="REGISTER"
                TextColor="White"
                HorizontalOptions="EndAndExpand"
                VerticalOptions="Center"
                Margin="0,0,10,0"
                Command="{ Binding RegisterCommand }" />
        </StackLayout>
        
        

        <!-- MODAL BACKGROUND LAYER -->
        <BoxView
            Grid.Row="0"
            Grid.RowSpan="3"
            BackgroundColor="Black"
            Opacity="0.8"
            IsVisible="{ Binding IsBusy }" />

        <ActivityIndicator
            Grid.Row="0"
            Grid.RowSpan="3"
            VerticalOptions="Center"
            HorizontalOptions="Center"
            Color="White"
            IsRunning="{ Binding IsBusy }" 
            IsVisible="{ Binding IsBusy }" />
        

        <!-- CANCEL BUTTON (appears after X seconds after register) -->
        <StackLayout
            Grid.Row="0"
            Grid.RowSpan="3"
            VerticalOptions="Center"
            TranslationY="100"
            Margin="50,0"
            IsVisible="{ Binding IsShowCancel }">
            <Label 
                Text="Still working...please wait!" 
                FontSize="Small" 
                TextColor="White"
                LineBreakMode="WordWrap"
                HorizontalOptions="Center"
                HorizontalTextAlignment="Center"/>
            <Button 
                Text="Cancel" 
                Command="{ Binding CancelRegisterCommand }" 
                HorizontalOptions="Center" />
        </StackLayout>
    </Grid>
</ContentPage>

RegisterPage.xaml.cs

using App.Core.ViewModels.Account;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace App.Core.Pages.Account
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
	public partial class RegisterPage : ContentPage
	{
		public RegisterPage ()
		{
			InitializeComponent ();
            BindingContext = new RegisterPageViewModel(Navigation);
        }
	}
}

RegisterPageViewModel.cs

using System;
using System.Threading.Tasks;
using System.Windows.Input;
using Xamarin.Forms;

namespace App.Core.ViewModels.Account
{
    public class RegisterPageViewModel : BasePageViewModel
    {
        private readonly INavigation Navigation;

        public RegisterPageViewModel(INavigation _navigation)
        {
            Navigation = _navigation;
            PageTitle = "Register";
            CheckEmailAddressesMatch();
        }

        #region Properties

        private string _fullName;
        public string FullName
        {
            get { return _fullName; }
            set { SetPropertyValue(ref _fullName, value); }
        }

        private string _email;
        public string Email
        {
            get { return _email; }
            set
            {
                if (SetPropertyValue(ref _email, value))
                {
                    this.CheckEmailAddressesMatch();
                    ((Command)RegisterCommand).ChangeCanExecute();
                }
            }
        }

        private string _confirmEmail;
        public string ConfirmEmail
        {
            get { return _confirmEmail; }
            set
            {
                if (SetPropertyValue(ref _confirmEmail, value))
                {
                    this.CheckEmailAddressesMatch();
                    ((Command)RegisterCommand).ChangeCanExecute();
                }
            }
        }

        private string _password;
        public string Password
        {
            get { return _password; }
            set
            {
                if (SetPropertyValue(ref _password, value))
                {
                    ((Command)RegisterCommand).ChangeCanExecute();
                }
            }
        }

        private bool _isEmailMatching;
        public bool IsEmailMatching
        {
            get { return _isEmailMatching; }
            set { SetPropertyValue(ref _isEmailMatching, value); }
        }


        private bool _isShowCancel;
        public bool IsShowCancel
        {
            get { return _isShowCancel; }
            set { SetPropertyValue(ref _isShowCancel, value); }
        }

        #endregion


        #region Commands

        private ICommand _registerCommand;
        public ICommand RegisterCommand
        {
            get { return _registerCommand = _registerCommand ?? new Command(RegisterAction, CanRegisterAction); }
        }

        private ICommand _cancelRegisterCommand;
        public ICommand CancelRegisterCommand
        {
            get { return _cancelRegisterCommand = _cancelRegisterCommand ?? new Command(CancelRegisterAction); }
        }

        private ICommand _backToLoginCommand;
        public ICommand BackToLoginCommand
        {
            get { return _backToLoginCommand = _backToLoginCommand ?? new Command(BackToLoginAction); }
        }

        #endregion


        #region Methods

        void CheckEmailAddressesMatch()
        {
            if (string.IsNullOrWhiteSpace(this.Email)
                || string.IsNullOrWhiteSpace(this.ConfirmEmail))
            {
                IsEmailMatching = true;
                return;
            }

            if (this.Email == this.ConfirmEmail)
            {
                IsEmailMatching = true;
                return;
            }

            IsEmailMatching = false;
        }

        bool CanRegisterAction()
        {
            //Perform your "Can Register?" logic here...

            if (string.IsNullOrWhiteSpace(this.Email)
                || string.IsNullOrWhiteSpace(this.ConfirmEmail)
                || string.IsNullOrWhiteSpace(this.Password))
                return false;

            if (!this.IsEmailMatching)
                return false;

            return true;
        }

        async void RegisterAction()
        {
            IsBusy = true;

            //TODO - perform your registration logic + navigate to the next page as applicable

            //Simulate an API call to show busy/progress indicator            
            Task.Delay(20000).ContinueWith((t) => { IsBusy = false; IsShowCancel = false; });

            //Show the Cancel button after X seconds
            Task.Delay(5000).ContinueWith((t) => IsShowCancel = true);
        }

        void CancelRegisterAction()
        {
            //TODO - perform cancellation logic

            IsBusy = false;
            IsShowCancel = false;
        }

        void BackToLoginAction()
        {
            //TODO - navigate to the login page
        }

        #endregion
    }
}