How to set up in Custom Map your own position?

How to set up in Custom Map (xaml) your own position? e.g. Warsaw

<local:CustomMap x:Name="customMap" Margin="2" IsShowingUser="true" MapClicked="OnMapClicked">

        </local:CustomMap>

I used Custom Map/Custom Pin/Custom Renderer
I have no idea how to set it to show Warsaw as standard
Sorry for my language