Starting from today, Turkish Airlines has removed the option to hold your booking ( a.k.a optional reservation or fare lock ) for free for a period and changed it similar to Lufthansa's hold system. From now on, passengers would be charged a fee, if they would like to hold a reservation, and the fee would be refunded after purchasing the ticket. If the ticket is not purchased, the hold fee would not be refunded. However, Classic Plus, Elite and Elite Plus members are exempt from the hold fee when booking a reservation on hold.
This is an image when trying to make a reservation on hold.
Note: The screenshot below was taken when I was not logged in to my M&S account.