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

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:PinClickDemo"
             x:Class="PinClickDemo.MainPage">

        <StackLayout>
            <local:ExtendedMap x:Name="Mapa"  Tap="Mapa_OnTap"/>
        </StackLayout>

</ContentPage>