It does ask for an annual pass number, if you have one, during the registration process.
Yes, you CAN AND SHOULD do this BEFORE you go. The QR code is simply a link to the website to use for reservations. You then have to figure it out from there. There was actually two QR codes. One guy scanned the smaller one inside the bulletin board case and it was the same link, but routed him to Dinosaur Valley State Park. He simply changed the park to Fairfield in a drag down menu. Same information at that point.
I say you should do this before, because it asks you to print out the reservation confirmation and pay receipt and bring it with you. I didn’t have a printer. I made pics of these with my phone to use...and it was
Fine though.