LoginPage.cs

    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class LoginPage : ContentPage
    {
        public LoginPage()
        {
            InitializeComponent();

            NavigationPage.SetHasNavigationBar(this, false);
            
            BindingContext = Activator.CreateInstance(Globals.LoginPageViewModel);

            UsernameEntry.Completed += (s, e) => PasswordEntry.Focus();
            PasswordEntry.Completed += (s, e) => LoginButton.Command.Execute(null);
        }

        protected override void OnAppearing()
        {
            base.OnAppearing();

            var loginPageViewModel = (LoginPageVm) BindingContext;

            if (!string.IsNullOrWhiteSpace(loginPageViewModel.Username) && !string.IsNullOrWhiteSpace(loginPageViewModel.Password))
                loginPageViewModel.CmdLogin.Execute(null);
            else
                UsernameEntry.Focus();
        }
    }

LoginPage.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"
             x:Class="Mobile.Core.Pages.LoginPage">

    <Grid VerticalOptions="Fill" Margin="0,20,0,0">

        <Grid.RowDefinitions>
            <RowDefinition Height="5*" />
            <RowDefinition Height="5*" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="30" />
            <ColumnDefinition />
            <ColumnDefinition Width="30" />
        </Grid.ColumnDefinitions>

        <Image Grid.Row="0" Source="loginlogo.png" Grid.ColumnSpan="3" HorizontalOptions="Center" VerticalOptions="Center" />

        <Grid WidthRequest="200" Grid.Row="1" Grid.Column="1" VerticalOptions="Center">

            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="30" />
                <ColumnDefinition />
            </Grid.ColumnDefinitions>

            <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
                <RowDefinition Height="Auto" />
                <RowDefinition Height="10" />
                <RowDefinition Height="65" />
            </Grid.RowDefinitions>

            <Label Grid.Row="0" Grid.Column="0" Margin="0,2" FontSize="24" Text="&#xf007;" FontFamily="FontAwesome" HorizontalOptions="Center" VerticalOptions="Center" />
            <Entry x:Name="UsernameEntry" Grid.Row="0" Grid.Column="1" HeightRequest="40" Margin="0,2" Placeholder="Username" Text="{Binding Username}" Keyboard="Email" />

            <Label FontSize="24" Grid.Row="1" Grid.Column="0" Margin="0,2" Text="&#xf023;" FontFamily="FontAwesome" HorizontalOptions="Center" VerticalOptions="Center" />
            <Entry x:Name="PasswordEntry" Grid.Row="1" Grid.Column="1" HeightRequest="40" Margin="0,2" IsPassword="True" Placeholder="Password" Text="{Binding Password}" />

            <ActivityIndicator IsRunning="{Binding LoggingIn}" Grid.Row="3" Grid.ColumnSpan="2" Grid.Column="0" VerticalOptions="Center" />
            <Button Text="LOGIN" x:Name="LoginButton" FontSize="14" Grid.Row="3" Grid.ColumnSpan="2" Grid.Column="0" VerticalOptions="Center" StyleClass="Success"
                    Command="{Binding CmdLogin}" IsVisible="{Binding LoggingIn, Converter={StaticResource BoolInverter}}">
                <Button.Style>
                    <Style TargetType="Button">
                        <Style.Triggers>
                            <DataTrigger TargetType="Button" Binding="{Binding IsEnabled, Source={x:Reference LoginButton}}" Value="False">
                                <Setter Property="TextColor" Value="White" />
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </Button.Style>
            </Button>

        </Grid>

        <Label Margin="0,0,0,10" Text="FORGOT PASSWORD" Grid.Row="2" FontSize="Small" HorizontalOptions="Center" VerticalOptions="Center" Grid.Column="0" Grid.ColumnSpan="3">
            <Label.GestureRecognizers>
                <TapGestureRecognizer Command="{Binding CmdForgotPassword}" />
            </Label.GestureRecognizers>
            <Label.TextColor>
                <OnPlatform x:TypeArguments="Color">
                    <On Platform="iOS" Value="#17B6B1" />
                    <On Platform="Android" Value="Accent" />
                </OnPlatform>
            </Label.TextColor>
        </Label>

    </Grid>

</ContentPage>