When we booked with Costco there was a space to enter our Fastbreak number, and it automatically linked to the Budget app. If you didn’t enter it there, Costco gives you both a Costco confirmation number and a Budget confirmation number, so you can go to the Budget website or app, enter the Budget confirmation number, and link it that way.
PS if you haven’t used Fastbreak before, your paperwork may not be in the kiosk in the garage, and if not you may still need to go to the counter in the garage to show your ID and credit card.