Add a tap handler to the map.
And have the handler as follows ::
private void MapControl_Tap(object sender, System.Windows.Input.GestureEventArgs e)
{
GeoCoordinate geocoordinate = this.MapControl.ConvertViewportPointToGeoCoordinate(e.GetPosition(this.MapControl));
}
You will have the map coordinates in the geocoordinate object. Hope i got the question right and the solution helps.