On sone circuits while on the start grid if its on a hill I need a way to be able to assign a button to hold the car to prevent faulse start There is an option in edit assignments "EXTRA BRAKE" but this does not work Any advice
Extra brake only applies to Mika Hakkinen's MP4/14, which had 2 brake pedals: the conventional one and the one used solely for steering the car. McLaren Racing - The search for the extra pedal
The button also works as handbrake in cars that feature it. Use your foot brake like the real life drivers do.
FYI, most vehicles in AMS2 don't have any functionality for the Extra Brake. I don't have a full list, but a good assumption is that street-legal cars with a license plate (TSI Cup, Street Supercars, Caterham…) have a handbrake, while roadcar-like race cars (Uno, Mini, Group A…) don't have a handbrake. I think the only exceptions to this rule at this moment are the ARC McLaren MP4/12 Not even the Lancer has a handbrake, which is a bit of a shame, especially when we get the Adrenaline DLC…
Actually Jean Ragnotti a french rally Driver Who's drove on track as well Asked His team to install a handbrake in His Renault 21. It was the first Time the car was There and he promise to himself if the car won the race it Will Do a 360. They spent the end of the last qual session not to setup the car but make the handbrake actually working on a car not designed for this.