MDE = My Disney Experience
It's an app and also exists as a subsection of the disneyworld .com website (
https://disneyworld.disney.go.com/). If you're using the website, you can sign in using your old username/email and password. There's a My Disney Experience drop down menu in the upper right corner of the homepage.
Click on My Reservations & Tickets. Allow the page to completely load.
Scroll down to Tickets & Passes. Click on Link Tickets.
Enter the Disney Reservation # (not the United Vacations Reservation #) in the box for ID number and click Next at the bottom of the page.
The ticket(s) that you ordered can now be assigned to anyone of your friends and family members who are listed in your MDE account.