Thanks both. It is a code share but I managed to find both the United and OS booking numbers. Both sites note free of charge seat booking option but still can’t do it. Neither site offers me the option to pay to book a seat. Anyway, I no longer am Senator on Star Alliance.

I will try calling the airline directly. Not a big deal if I have to take what I can get at check in but I do try to maximize benefits where I can 😁