Find your way to Coney Island and go to the New York Aquarium.

From there, follow the path that leads up to the boardwalk.  Stop at the gate and look to your left.

Pretend to tie your shoe and reach down under the tubular support in the corner closest to the Aquarium (when this box was planted there was a brown trash can here), there you will find a tiny magnetic box affixed beneath.

PLEASE be very careful of who is watching.  This area is very, very busy (it is New York, and it is summer at Coney Island!).  Move out of the area to stamp in and then return to carefully replant.