use polygon for an overpass turbo query

Edit: You can't import data into overpass & it doesn't really understand true polygons, just closed ways, but you can use existing OSM data.

There's quite a few ways to achieve this depending on what data you have for the area to be searched:

{{geocodeArea:Greater London}};
rel(pivot)->.Bound; //change to way to render
way(r.Bound)->.Bound;
.Bound out geom;
nwr[amenity=police](area);
out center;

These three lines are only to display the search area's boundary & can be removed if desired:

rel(pivot)->.Bound; //change to way to render
way(r.Bound)->.Bound;
.Bound out geom;

The boundary's ID:

rel(175342); map_to_area;
nwr[amenity=police](area);
out center;

The IDs for relations start at 3600000000 so by adding the id value to it allows

area(3600175342);
nwr[amenity=police](area);
out center;

Note the different brackets used:

area[name='Greater London'];
nwr[amenity=police](area);
out center;