Example of how to add pins in maps with click in Xamarin.Forms applications. - http://julianocustodio.com/adicione-pontos-no-maps

ExtendedMapRenderer.cs

using Android.Gms.Maps;
using PinClickDemo;
using PinClickDemo.Droid;
using Xamarin.Forms;
using Xamarin.Forms.Maps;
using Xamarin.Forms.Maps.Android;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(ExtendedMap), typeof(ExtendedMapRenderer))]

namespace PinClickDemo.Droid
{
    public class ExtendedMapRenderer : MapRenderer, IOnMapReadyCallback
    {
        private GoogleMap _map;

        public ExtendedMapRenderer()
        {
        }

        protected override void OnMapReady(GoogleMap googleMap)
        {
            _map = googleMap;
            if (_map != null)
            {
                _map.MapClick += googleMap_MapClick;
            }
        }

        protected override void OnElementChanged(ElementChangedEventArgs<Map> e)
        {
            if (_map != null)
            {
                _map.MapClick -= googleMap_MapClick;
            }

            base.OnElementChanged(e);
            if (Control != null)
                ((MapView)Control).GetMapAsync(this);
        }

        private void googleMap_MapClick(object sender, GoogleMap.MapClickEventArgs e)
        {
            ((ExtendedMap)Element).OnTap(new Position(e.Point.Latitude, e.Point.Longitude));
        }
    }
}