CustomButton.cs

using Android.Content;
using Android.Content.Res;
using Android.Graphics.Drawables;
using System;
using System.Threading.Tasks;
using YourProject.Droid.Renderers;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(Button), typeof(CustomButtonRenderer))]
namespace YourProject.Droid.Renderers
{
  public class CustomButtonRenderer : ButtonRenderer
  {
    public CustomButtonRenderer(Context context) : base(context)
    {
    }
    
    protected override void OnElementChanged(ElementChangedEventArgs<Button> e)
    {
      base.OnElementChanged(e);
      //get the button from XAML file
      var button = Element as Button;
      
      //create a new shape of the button with used BackgroundColor
      var shape = new PaintDrawable(button.BackgroundColor.ToAndroid());
      
      //set the CornerRadius property 
      //the Xamarin.Forms CornerRadius uses different unit than native Android CornerRadius,
      //that's why there is *4 (you can experiment and find a better solution)
      shape.SetCornerRadius(button.CornerRadius * 4);
      
      //create a RippleDrawable out of the previous shape where is possible to add the right ripple effect 
      //(I used LightGray - it's possible to use whatever you want, even taking the color from the XAML button)
      var ripple = new RippleDrawable(ColorStateList.ValueOf(Android.Graphics.Color.LightGray), shape, null);
      
      //set the ripple as the native button's background 
      Control.SetBackground(ripple);
      
      //this will remove the shadow behind the native button
      Control.StateListAnimator = null;
    }
  }
}