Highlight specific locality/state from an address

Hey! currently Im getting a position from an address (Example: the user put Manhattan and i get te latitude and longitude of the center of manhattan). So far so good .. but now I want to highlight all manhattan area, but I dont know how to do it automatically, without hardcode. I am using Xamarin.Maps and Plugin.Geolocator. Also, for the polygons i used the "Highlighting a Region on a Map" guide from xamarin doc. I want to highlight all manhattan area without hardcoding the shapeCoordinates, as google maps do.

Thank You!