Originally Posted by DaveSFO

I do it under the online seat assignment. You can use it either for selecting seats or checking in. My flights are in July and I had no problem selecting seats via this route. I just put in my itinerary information and it comes up. I can even use the 016 ticket number on my eticket receipt. This is my third award reservation with them and I have been able to do this each time.

I'm not sure about the codeshare issue, but it wouldn't surprise me if that is what's causing them to say you have to do it at checkin. I've never booked a codeshare with them so I can't say for certain.