I have a ticket purchased from United from SFO - AMS - IST in business class. AMS - IST is operated by Turkish. I have the TK confirmation number and I am able to look up the reservation on their website, but I am unable to pick a seat. I called TK and they are able to see the ticket as well too, but their computer is unable to choose a seat for me. The agent says because it is a UA flight number even though it is operated by TK. I called United and asked for help and they were not able to do anything. Does anyone else have any suggestions?