I'm trying to do the same thing as the original poster in addition to adding my passport and immigration information, phone number and e-mail address, however it appears I am unable to edit any fields. When I click on the "Passenger information" tab after loading my reservation details, I see the message "Please register the required information for online check-in. " at the top of the screen. In the reservation details, I see fields such as "Email Address Please register"but with no clear way to edit them. I've tried different browsers, turning off uBlock Origin, Incognito mode, etc and nothing seems to work.

There is a link next to the "Please register the required information for online check-in. " message (the little "I" icon) which takes me to the below screen, but after entering in my details it takes me back to the original page.

I booked this through United as a codeshare, so I'm wondering if that's the reason for this not working. All of my details are entered already on the United booking, including my passport information.