Example of Push Notifications with OneSignal in Xamarin.Forms applications - http://julianocustodio.com/onesignal-push-notification

MainActivity.cs

...

        protected override void OnCreate(Bundle bundle)
        {
            TabLayoutResource = Resource.Layout.Tabbar;
            ToolbarResource = Resource.Layout.Toolbar;

            base.OnCreate(bundle);

            OneSignal.Current.StartInit("Insert OneSignal App ID here").EndInit();

            global::Xamarin.Forms.Forms.Init(this, bundle);
            LoadApplication(new App());
        }

...