Where's the AppDelegate Window object

I'm trying to add a general behavior that puts an overlay over the entire screen on any screen.
On iOS I was thinking about using the Window (AppDelegate.Window), but it seems that the Window doesn't exist on Xamarin.Forms:

// Triggers a ModelNotImplementedException
UIApplication.SharedApplication.Delegate.Window

How can I get a reference to the topmost view from within another view?
Or are there any more suggestions to add my full screen overlay?

thanks,
Thomas