I did this last year for the same reason you're asking: I had to return to the airport to pick up a rental car. I had a relatively small time frame in which I wanted to arrive at the airport, though, so it took a tiny bit of research.
DME will pick you up at your resort about 3 hours before a domestic flight is scheduled to depart, and it takes about 30-40 minutes to get to MCO from WDW. So, last year, I needed to arrive at MCO at 2:30pm. My family arrived right around that time, and I wanted to have the car rented by the time they retrieved their bags. Since I wanted to arrive at MCO at 2:30, I knew I'd need to be picked up around 2:00 pm. Since they pick up 3 hours before a domestic flight, I just went on American Airlines website (though you could pick any airline), and just searched for a flight from MCO to DFW (I picked that because there are many flights to DFW from MCO each day) that departed close to 5:00 pm. I jotted down the flight number and gave it to the DME reservation agent. There were no additional questions, and there's nothing else that needed to happen beyond giving them the flight number. Once you give them that, they can see the flight time, and the reservation time will be about 3 hours prior to that.