Originally Posted by tods27

Call UA and ask for the SN Record Locater (PNR) - you used to be able to get this in My Trips on UA.com, but it seems to have disappeared again. Then go to the SN website (or better yet, their smartphone app), and go to "Plan and Manage" > Retrieve Booking > Lookup Single Itinerary. Enter your data, then when the reservation comes up, click "Edit Booking" and there is an option to "Enter Entry Data".

In the SN App, you have to pull up the reservation, click on one of the legs, then you get an option to "Manage my Booking". Scroll down to "Passengers" and open the details, then select "Modify" and you can enter your Passport information there.