Try this, instructions are in Turkish and English on each page, it should be done by first named person on logbook :
1. May only work if you registered your details by text to 1300 in July time
2. Go to this website
https://bub.gov.ct.tr/en-us/
3. Scroll down to see the AKOS online payment and click it
4. Follow the instructions: input reg no., passport number used when purchasing the vehicle preceded by a P, date of registration from front of log book and the verification code and submit
5. If that works it should bring up your car details, so you can do that process anytime.
6. On Lhs, the first option should be to pay tax, click on that
7. Next page should show one line with all the details, when due, how much, etc. There is a tick box that you must complete then click submit.
8. Complete your payment details and if successful, it will tell you to print out your tax disc, you can also save a copy. The printed tax disc should be kept in the car.
Good luck and sorry if I've got anything wrong, but really it's self explanatory.