custom-android-entryrenderer.cs

public class CustomAndroidEntryRenderer : EntryRenderer
{
	protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
	{
		base.OnElementChanged(e);
		
		if (e.OldElement == null) {
			// Get the 'native' control
			var nativeTextField = (EditText)Control;
			
			nativeTextField.Text = "Ahoy there from Android!";
		}
	}
}

custom-entry-instantiation.cs

new CustomEntry
{
  Text = "Space Pants"
}

custom-entry.cs

public class CustomEntry : Entry {}

custom-ios-entryrenderer.cs

public class CustomIOSEntryRenderer : EntryRenderer
{
	protected override void OnElementChanged(ElementChangedEventArgs<Entry> e)
	{
		base.OnElementChanged(e);
		
		if (e.OldElement == null) {
			// Get the 'native' control
			var nativeTextField = (UITextField)Control;
			
			nativeTextField.Text = "Ahoy there from iOS!";
		}
	}
}

example-layout.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:local="clr-namespace:CustomEntryRenderer;assembly=YourAssembly"
    x:Class="CustomEntryRenderer.MainPageXaml">
    <StackLayout 
        VerticalOptions="CenterAndExpand"
        HorizontalOptions="CenterAndExpand">
        <Label Text="Hello, Custom Renderer!" />
        <local:CustomEntry Text="In Shared Xaml" />
    </StackLayout>
</ContentPage>

main-page.cs

public class MainPage : ContentPage
{
	public MainPage()
	{
		Content = new StackLayout
		{
			Children =
			{
	        		new Entry
	        		{
					Text = "Ahoy there!"
				},
				new Button
				{
					Text = "Login",
					TextColor = Color.White,
					BackgroundColor = Color.FromHex("77D065")
				}
			},
			VerticalOptions = LayoutOptions.CenterAndExpand,
			HorizontalOptions = LayoutOptions.CenterAndExpand
		};
	}
}

renderer-attribute.cs

[assembly: ExportRenderer(typeof (CustomEntry), typeof(CustomEntryRenderer))]