I just had a rental with Sixt in Dublin. Quite frustratingly you can't enter a Sixt loyalty number on Priceline when you book it, but on Hertz, Avis, Budget it's no problem. Priceline also doesn't push the email address to Sixt so you can't even go to their website to add it since their website requires your email in the reservation to pull it up.

This is a huge downside since Sixt will still give you an upgrade at the counter once you show your Platinum number but since they didn't know a Plat member was coming, they didn't arrange for the car to be in the terminal parking vs. the remote location that requires a bus ride. I also got the impression that they didn't have much upgrade inventory when I arrived and the agent inferred I'd have gotten a better upgrade if it was in the reservation.

I know with Avis and Budget it's quite easy to see the discount code once you've made a reservation, but Sixt has a more complicated system, you need both the discount code and a code key on their site, how can one get that information to use the Autoslash discount AND have the Sixt loyalty number in the reservation?

Alternatively can you work with Priceline to get the loyalty information box added like they do for other rental companies? And have them push the email address in the reservation so a customer can access it on the Sixt site?