Managed to do seat selection in a complex way. I booked with an OTA and I didn not succeed in opening manage your booking on any non-China AC webside. So I went to https://www.airchina.com.cn/, used the translate function of MS Edge, then chose for the guest login function with ticket number and one time password sent to your phone. Then could find my booking and select seats, as reported for free in the back of the plane, things in front cost.

I was not able to find any way to add my *A gold FF status with TK, unfortunately so can't tell if that gives you more seats to select. The call centre I called said I must do this at check-in at the airport.........