How to properly authenticate user with Linkedin for both IOS and Android platforms

Hello. I have been looking around to implement login with Linkedin. I have become confused since I have found out that many people use OAuth to authenticate (such as https://forums.xamarin.com/discussion/14883/login-with-linkedin) but the GetUI function of the OAuth only works in Android and not Xamarin forms:

    var intent = auth.GetUI(ctx);
            StartActivity(intent);

Also the StartActivity is Android specific. How can I authenticate in PCL solution or replace the code above to fit for IOS as well?