At the budget site, just click on reservations and fill in your info. At the bottom of the page there is a box for codes. Sorry I'm trying to tell you this from memory so I'm not 100% sure, but I think that's where it is.
I had reserved a van through Dollar at the 209.00 + 39.99 + taxes and fees, but after reading the priceline success stories I bid $24/day on a mini van and was accepted at Budget. This is for 8 days in June for a grand total of $247.??.