AC will allow you to select a seat- you'll need to call them. AC IT is horrible, so if booked through UA, you'll have to call, AC web site won't allow it. Some have reported tweeting to AC will help.

Do note that if the fare class you book translates to one of the tango fare classes (the few lowest), you'll need to pay if you want seat assignments in advance. They should be able to do it, but do note it would cost - cost is typically based on route length.