You would need to call when the discount is released to see if it can be applied to your reservation. They only allow a certain number of rooms per discount, and some room types are not included. If the discount can be applied, they will refund you any money owed, or just apply the amount of the discount toward your final balance.
You have to call, it won't happen automatically because they are trying to sell more rooms not refund people. And as PP says, they will only do this if your room category is included in the discount and you meet any other required criteria such as arrival dates.
We often book room only reservations and they'll charge a one night deposit. Then we keep an eye out for any discounts that may eventually come (AAA, AP, etc) , then call and if they're available they will credit the difference in our deposit to the balance of our stay.
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