Example how to monetize yours Xamarin.Forms applications with AdMob by Google - http://julianocustodio.com/admob-xamarin-forms

AdMobView.cs

using Xamarin.Forms;

namespace DemoAdMob
{
	public class AdMobView : View
	{
		public static readonly BindableProperty AdUnitIdProperty = BindableProperty.Create(
			nameof(AdUnitId),
			typeof(string),
			typeof(AdMobView),
			string.Empty);

		public string AdUnitId
		{
			get => (string)GetValue(AdUnitIdProperty);
			set => SetValue(AdUnitIdProperty, value);
		}
	}
}