On National's website, I put my information in on the right side of the screen. Towards the bottom, after it asks for dates, place, type of vehicle, it will ask you to plug in any discounts. Specifically it asks for:
1. Contract ID
2. Product rate code
3. Coupon ID
I listed mine in a previous post. Mousesavers.com gives MANY ideas and combinations as to the various codes you can use. Sometimes you just need to play around a bit to find out which work best for you.
The contract ID 6600330 is from priceless vacations. There is a wesite....but you don't need to sign up to use their code. There is a thread on the transp. board that talks about this topic.