This is how I usually approach using codes:
I ask to check pricing and availability for arrival on __________ for ___ nights staying at __________ Resort. I ask if they would please check for a discount using the code ____. They will usually ask for the party size and from there they will check on the resort you request with the code. If it is available, they will most likely quote a total for the entire stay (including tax) instead of a nightly rate. If the discount isn't available, they will let you know.
I hope this helps!