interstitialAd = new InterstitialAd("ca-app-pub-xxxxxxxxxx/xxxxxxxxxx");
interstitialAd.ReceivedAd += OnAdReceived;
interstitialAd.FailedToReceiveAd += OnFailedToReceiveAd;
interstitialAd.DismissingOverlay += OnDismissingOverlay;
AdRequest adRequest = new AdRequest();
adRequest.ForceTesting = false;
interstitialAd.LoadAd(adRequest);
showInterstitial.IsEnabled = false;
and to Show :
interstitialAd.ShowAd();