I would enter a coupon code, any code, and then use the one you want when you get there. The reason is that almost all coupon codes have blackout days, restrictions, location does not participate, etc. When you make the reservation, if the coupon is rejected, it tells you immediately.
If you have a code "plugged in" and accepted, at least you know that it will be honored. You can use any coupon you want once you get there. The coupons are never actually entered until the contract is entered.