Thank you for the replies. I called up local Turkish airlines and was told that they only serve to M&S members and recommended us to register as members. I have also read that due to its popularity, TK is now only offering the cakes to M&S members.

Yes, I am thinking of changing the FFP when doing online checkin (if available) or at the airport (which I do not like, sometimes the agents just make mistake and do not put in the FFP numbers correctly)