I found that I had to dummy book all the way (using fake details) until the page where it says confirmation details and then requesting you to select seat (at least for Biz class) for your respective sectors. One of the entries (for each flight sector) would be Class followed by a letter presumably the booking fare class. I think for Restricted Economy it's usually S(?), at least from what I remember while for Business at least those I've dummied so far, were D.

Up to that point, you don't need to enter any credit card details and all the info you enter up to that point can be fake assuming you don't intend to proceed further. The only possible sensitive info (I don't know if you can dummy it or not) is some tax number thing that applies only to Turkish citizens I think. I never filled that in.