The first time, I made an ADR for 8 people for a date in April 2007 for 8. I made the reservation at 180 days out, and didn't say I had the DDE. I called back a couple of months later and I got a nice CM who credited my credit card back with the 20% discount.
The next time, for an ADR for 7 people in October 2007, my friend made the ADR for our two families with her credit card, but I am the one with the DDE card. We both tried to get the discount applied with two different phone calls, but that time they told us to tell our server and we will receive a gift card for the 20% discount at the restaurant. Maybe because it was too complicated with an ADR in one name, but the discount card in another. It was fine. We told the server and he took the card and disappeared. It took him a little time to get it done and when he returned he told me that next time I had to do this to tell the server at the beginning of the meal.
Good luck