Triangle on PS4, Y on XBOX. You must be controlling the returner to signal it, so if you aren't controlling him at the snap you need to switch onto him.
Here's a bit of bonus info too - if you control the returner at the snap and then press nothing, he will try to catch the punt no matter where it lands - even if it's within 10 yards of the endzone. If you want to let it bounce and hope for a touchback, you either need to move the returner out of the way manually, or not switch onto him until you're sure the ball will go into the endzone. While he's CPU controlled, the returner will peel off the ball and not try to catch it if it's close to the endzone.