(Xamarin.Auth) Sign-in On Authentication Cancelled

Hi there, I'm new to Xamarin.
I want to know when user cancelled the process. (the back button from the embedded browser).

//This section opens an embedded browser
Android.Content.Intent intent = GoogleOAuthManager.Authenticator.GetUI((Android.App.Activity)Forms.Context);
Forms.Context.StartActivity(intent);

//This checks for authorization.
GoogleOAuthManager.Authenticator.Completed += OnAuthenticatorCompleted;

The Completed event fired successfully.
In Xamarin Auth, there's an OnCancelled method.

namespace Xamarin.Auth
{
    public abstract class Authenticator
    {
        public Authenticator();
    ...
    public void OnCancelled();
    ...
}}

It's not a virtual method, so I can't override it?