Don't call ShowRewardedInterstitial just after LoadRewardedInterstitial because you need to give time to load the interstitial.
There are events you can use to check when the ad has been loaded.

Where you load an interstitial, is up to your logic, OnAppearing might be ok if this is what you prefer for your app

Hello. Moved to

  public AboutPage()
  {
     InitializeComponent();

     CrossMTAdmob.Current.LoadRewardedInterstitial("ca-app-pub-xxxxxx");
}

  private async void Button2_Clicked(object sender, EventArgs e)
  {
     CrossMTAdmob.Current.ShowRewardedInterstitial();
 }

}

On emulator nothing happends but said in console: Reward not loaded
Thx

@marcojak the package readme contains They are implemented but currently they are not working. Probably something in the Admob SDK. I'm investigating it. for Rewarded Interstitial.
Is this already resolved ?