That did the trick. I just assumed the SAS PNR was used for all legs as no TK PNR was found. Following your advice I used CheckMyTrip, found the TK PNR and was able to make a seat reservation using the app previously suggested. Trying to use the TK website did not work, I just got a message that this booking was done using a partner and I had to contact the partner.
So now I was able to select seats for all TK legs. sweet The new TK BC cabin on the Dreamliner looks really nice.

Thanks to everyone here for helping out. This is my first award trip, I made my first BC trip in 2017 so quite new to this