using GoogleMobileAds.Api;
...
// Initialize an InterstitialAd.
InterstitialAd interstitial = new InterstitialAd("MY_AD_UNIT_ID");
// Create an empty ad request.
AdRequest request = new AdRequest.Builder().Build();
// Load the interstitial with the request.
interstitial.LoadAd(request);
if (interstitial.IsLoaded()) {
interstitial.Show();
}