1. Buy 3 credits (can't buy 2) form this seller that Luke301 recommended to me for 6,60 usd total: animeman65
(2. If you don't know how to flash official samsung firmware (which will not trigger KNOX), check: XDA: How to update the firmware on any Samsung phone)
3. Flash official samsung OXM THL firmware I downloaded with Frija (not sure if necessary but in the Samkey application it says you need to do this first). You can also use sammobile but downloading takes very long (6 hours).
4. Enable developer options on your phone: Settings > About phone > Software Information > Tap "build number" 8 times or until you see a pop-up message that says "Developer mode has been enabled".
Disable any lock screen or lock pattern (fingerprints, iris scan, etc...)
5. Turn on usb debugging on your phone: Settings > Developer options > USB debugging > Allow USB Debugging > Tap "OK"
6. Install and open the Samkey application on your computer > Connect your phone to your computer with a usb cable > Login witch the credentials you received from the ebay seller.
7. Insert the CNC code you want (in this case THL that has native call recording) into the "Change CSC (Buyer)" field > Click the "Change CSC" button > Confirm that you want to spend 2 credits.
8. Let the application do it's work after which it will reboot and then you're good to go.
Very important: Samkey doesn't work with android 10 beta, so you need to change csc in pie and then upgrade without factory reset.