Get access to View Controller

Hi - I'm using Xamarin Forms and using Azure Mobile Services to Authenticate. During this process the MobileService.LoginAsync() method is called to authenticate. However I want it to pass a UIViewController, however I cannot get back to the UIViewController that it came from.

Any examples or code snippets might help, I'm also already using a custom Button Renderer thinking I can get back to the UIViewController When I Click Login Using Facebook Button.