Xamarin Auth: Display loading message while webpage is loading

I'm developing an app using Xamarin, and I'm trying to use Xamarin.Auth to add Facebook authentication. Everything works fine and I manage to connect successfully.

However, when you don't have Facebook app on your device, it uses the webpage version. While downloading the page, it only shows a blank screen and seems like it has crashed (but it's not). I want to display a loading message while the webpage is downloading to let the user know that it didn't crash at all and he has to wait a little bit and to dismiss it when the download is complete.

Thanks for reading and I hope that you can help me out...