Each airline on a booking tytpically has its own PNR. I'm actually very surprised that BR used the same PNR as United.

If you bring up your reservation on united.com, there should be an option to see CA's PNR. You should then be able to use this on CA's site. The other option would be just to call them, in which case you don't need the actual PNR - just your flight information should suffice.