cannot convert from 'Android.Content.Context' to 'Android.App.Activity'

Hi, I'm implementing authentication in a Xamarin.Forms app by using https://developer.xamarin.com/guides/xamarin-forms/cloud-services/authentication/oauth/ and my OAuthLoginPresenter is like
public class OAuthLoginPresenter
{
public void Login(Authenticator authenticator)
{
Xamarin.Forms.Forms.Context.StartActivity(authenticator.GetUI(Xamarin.Forms.Forms.Context));
}
}
it is showing cannot convert from 'Android.Content.Context' to 'Android.App.Activity' error.Help me for solving this.
Thnak you.