I didn't have my flights booked when I booked my resort reservation, so I skipped booking Magical Express. In the past, I had to call Disney to book DME. This time, I was able to go into the Disney website to change my resort reservation, and from there I could book DME and enter my flight info! Maybe others know this, but I thought I'd post in case it's helpful to someone. I'm ~80 days away from arrival, so I'm not sure how if there's a point at which you can't change the reservation without some impact (so YMMV).
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