You do not get to request a room assignment during online check in. You should call reservations to add your room request on your reservation before doing online check in.
The benefit to online check in is that your packet is made up for you and you can use the separate online check in line at the front desk, which is usually shorter and faster than the standard check in line.
In my experience when I've used online check in, my room is almost always ready but does not meet my room requests. The impression I got from a front desk CM was that they try to place online check in people in rooms that will be ready, which is why they ask for your approximate arrival time when you do online check in. We did not get our room request when doing online check in, so IMO, they would rather place you in a room that is ready when you arrive than a room that isn't ready but grants you your request. So if you add room requests, it's my recommendation to not do online check in.