At the first screen under "Promotion#" enter 'BCFD'. In my experience, the system usually erases the promo number anyways, and the booking is always priced out as per normal. You need to bring the paper certificate in with you anyways, so whatever happens with the booking, just show them the award certificate and they will adjust it.

It's far from a simple and smooth process but free is free. I wish they'd switch to National's all-electronic system. You get credited the award the instant you earn it, the confirmation shows the post-award price, and I've never had a problem at the rental counter.