FYI, UA codeshares that are operated by AC now have seat selection available on UA.com. This is pretty new, I think. Just booked a flight like this (on UA.ca, though) a couple of days ago, and was surprised to be able to do this. Seat map wouldn't show up during the booking process, and was assigned a random seat in the back, but logged in after booking, and was able to select my own seat (to my surprise). Called AC, and they confirmed the new seats.

Next time, even on a TA booking, you should be able to do this by getting the UA booking reference, then changing in your itinerary by logging in with this and your last name.