I was wondering who I needed to contact regarding my magical express reservation when I booked my package through a 3rd party online.
I tried calling the Disney number but they said they couldn't make any changes. Should I call Magic Express directly or try calling WestJet (who I booked through)?
Call the place you booked your trip through. If Disney is saying you can't make changes yourself, Magical Express will have the same answer.
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What Allie said 
You may want to call the resort you are staying at. I have booked rooms thru a 3rd party and as long as I had the reservation number I was able to make a magical express reservation. There was a time you could add magical express online with your hotel reservation number, but not sure if that still exists.
We figured it out! WestJet sent me the link. There is a separate Magical Express website (that does not work in Safari as I learned)
https://disneyworld.disney.go.com/airport-transportation-reservation/
Cool.....this is the same online form I used to sign up for Magical Express through the DVC site.