Unfortunately to gain this achievement you are required to complete the game and pick up the Rex token during the final Chapter.
Once you have done this find your closest Character Booth and purchase him which I believe is 1,000,000 studs and select him with your Chase McCain Robber slot with the
Now head over to Crescent Park location in below image(s):
Once you are in the park you should notice two if not three LEGO city AI on Lawnmowers, which will be riding around the park in circles.
Walk over the the pathway and wait for them to loop around when close press
to activate Chase McCains Whistle and Badge which will stop them.
Now just walk over and when prompted press
to gain access now just press
to drive and wait for that magic pop!
Please see my game clip for clarity:
Sorry, this Game Clip has been removed