I called AC Aeroplane because it's not possible to see TK PNR on their website.
They gave me TK PNR and added my TK# to the reservation.
After that I could see the reservation on TK website, but still could not assign a seat (it shows an error saying that this is an external booking). I tried it both when logged in and logged out.

However, assigning a seat worked fine in TK App.
It's weird that their App is better than their website.