I've had a heck of a time with this and the manual for PC, from Steam, leaves a lot to be desired.
Start a career, go into the trailer with the agent. Arrow over one from here for the "enter the race" setup screen. Choose Expert. Choosing expert is the only way I can get to the full setup menu to customize it. Once you're in the expert setup screen you can arrow up and down to customize the whole race venue such as tires, fuel, ledgend AI, and many other things.
Good luck!