You also asked this question in another thread. I answered it there, but I'll answer it again here. (It was off-topic in the other thread anyway.)
Three different different solutions to consider:
- Instead of using an online form, talk to a human being at Disney's Magical Express (DME) Guest Services at 866-599-0951. They should be able to make a reservation for you regardless of how you're arriving at Orlando International (including by rental car).
- Look for a flight that arrives around the time you expect to arrive at Orlando International and provide that flight information, even though you won't be on it. It's not really cheating. DME is a service to get you from the Orlando International to your WDW resort. It has nothing to do with how you arrive at the airport.
- Look into changing your car rental reservation to return your car at a location in or near WDW. Arrive at your resort by car. Then, at your leisure, return your car to a local location near WDW; they'll take you back to your resort. You may even want to keep your car for a full week if you can get a good weekly rate and you're first spending a number of days around Fort Lauderdale. Cancel your arrival day DME reservation. Make a DME reservation only for your check-out day.
I think the third choice is the best choice. You're paying for a one-way car rental anyway, so there may not be much price difference. It should save you a lot of time.