In my experience, when you check in, there is an option to add your number, but it turns out that if your trusted traveler number is in your profile, then you do not need to do that and it will correctly populate. If you get all the way through and you don't get pre-check, the app should tell you that, in which case you can call United, make sure all the names match, have them uncheck you in and re-check you in and you should be fine unless you were randomly selected for the dreaded SSSS.