It depends on what throttle hardware you have. There are commands called Set Engine 1/2 Fuel Valve that you can use. If your throttle has a virtual button on the IDLE position, you can Set that virtual button to be bound to that Set Engine 1/2 Fuel Valve but use it with “On Release”. That way, the fuel valve will still be on at any throttle position other than IDLE, but as soon as the IDLE virtual button is “pressed” it actually release the fuel valve command and cuts off the fuel flow.
If you don’t have an virtual button on the idle detent, there’s also usually a virtual button tied to the throttle Axis, like Axis X- and Axis X+ where one of them will be assigned to each end of the throttle axis. Just bind the one that is sitting on the IDLE throttle, and you should be able to do the same logic as above.