Bit of a tangle here. I used Lifemiles (Avianca site) to book a flight, TPA-ORD-IST-BEY. I received a PNR from Avianca, which I fed into the UA site, and up pops my United PNR. No seat assignment with TK, so I go to Turkish website, but they don't accept United PNR; they have their own. I called Turkish and was told that I have to contact UA to get the TK PNR. So I did. However, UA said that I have to call Avianca (Lifemiles) to get the TK PNR. So, I called Lifemiles and was told, no, it's United that has the PNR for TK.
In short: I have an Avianca PNR that allows me to enter the UA site and pull up all my flights, but no way to choose seat assignment on TK; for that I have to go to TK directly (with TK PNR, or so they say). Who has access to my ability to choose seats on TK? Certainly not me!