Mine worked. Go to the Ubi soft store, sign in with your Ubi account first (trust me, don't do it during checkout or things can go all bat-crazy). Once there, put HAWX2 in your cart, then use the voucher code as the promo/coupon code in the cart. It should reduce the cost to zero, and you shouldn't be prompted for credit card info if you don't have anything else in your cart.
It might take a while to get the cd key emailed to you though. I think mine might have taken half an hour or more during normal business hours.