This example show native messages to users in Xamarin.Forms applications - http://julianocustodio.com/native-message

MainPage.xaml.cs

using System;
using Xamarin.Forms;

namespace DemoNativeMessage
{
	public partial class MainPage : ContentPage
	{
		public MainPage()
		{
		    InitializeComponent();
		}

	        private void Button_OnClicked(object sender, EventArgs e)
	        {
	            DependencyService.Get<IMessage>().LongAlert("Hello");
	        }
        }
}