Originally Posted by dr_torch

Once you book with UA you should be able to get the AC PNR from the UA website. Use that to select seats on the AC website. Exit row seats and seats with extra room are called "preferred seats" by AC. For these you will pay a fee, but standard seats should be free.

I'm not sure about 24 hours before. I have seen cases where those seats were still paid for when using OLCI and I have seen cases where there were free. I'm not sure what the official policy is there.