Just enter in a made up model and license plate. They only want the info in case they have a pic of you going through without it registering so that they can easily match it up.
They say that when you rent a car, you are supposed to register that cars license plate to your tag 24 hours before you use it. Problem is you don't usually know the license plate of a car until you rent it. So you just don't register it and everything is fine. The only time it helps you to register it is if for some reason the E-Z Pass reader doesn't catch it, and they have a pic of your license plate, when registered they will simply match the plate to your account and charge your account the toll. If unregistered it takes them more work to track you down (contacting rental company etc.) so E-Z Pass will charge you $25 fee, and usually the rental company will charge a fee as well.