I suspect you’re correct. That said, if OP does not have a MileagePlus credit card but wants to get one, I believe the must-use-the-card rule is still waived for 90 days after initial signup. OP would still need to get the MileagePlus number on the reservation, and even then I can’t promise it would work for a non-UA ticket, although I suspect it probably would. A buy-up on the day of departure, or just paying the bag fees, would be the simplest approaches.

OP: You will get a full-size carry-on plus a personal item, BTW. UA’s restrictions on carry-ons that you might hear about in the news only apply to Basic Economy, and award tickets don’t book into BE.