I couldn't find it. Regardless, the price in the app was already higher than the website. With the 15% you will get the original price.
When you go to the payment option, there is a place where it says "I have a promo code".
Installed the app and tried to check the the price. First, the price was higher than in the website. Second, I couldn't find a place to enter the code. Cold for me!
Watch out for journeys with Turkish over the next few months, their main hub airport is going to move to a new facility and there is likely to be some snafus