var oAuth = new OAuth2WebServerFlow(ClientId, ClientSecret, CallbackUrl, Scope, 
                                    AuthorizationUrl, AccessTokenUrl);
                                    
OAuthUiBroker.ShowOAuthView(oAuth);

OAuthUiBroker.OnAuthenticated += async (sender, args) => {
  await Application.Current.MainPage.DisplayAlert("Message", "Access Token: " + args.AccessToken + 
                          "\nRefreshToken: " + args.RefreshToken + 
                          "\nExpires In: " + args.ExpiresIn, "OK");
}