I've just got this achievement with the Atlanta Falcons. (WR - R.White) You don't have to be playing away as stated in another solution and also you don't have to let the opposition score before doing this as it was 0-21. After running into the End Zone unchallenged i ran into the blue square under the posts and pressed B. But as confirmed by "canica" in a comment you don't need to be inside one of the blue squares. The achievement pops as soon as you press B.