There's very few cases when AC will allow you to select seats online if not booked on Air Canada's web site. That includes, at least until fairly recently, not sure if it's till the case, Aeroplan award tickets. So unfortunately, this will likely require a call to AC. This was supposed to be fixed a couple of years ago, but as I understand it, still isn't.

Note that AC also has terrible hold times even on a good day on the non-elite line. I would expect to budget 45 minutes at least for that. Trying off hours (I.e. Middle of he night) YYZ time is probably your best shot at a shorter wait.