I’m checking the command list through the alternative launcher and I see these two:
TQS: Cutoff Release: Idle Detent - Idle
TQS: Cutoff Release: Idle Detent - Off
They also seem to be mapped in the default warthog profile (check the hotas folder inside your BMS install)
Using the AL (which personally I would recommend), Off is mapped as press (as in, moving the throttle backwards past the detent, engaging the DirectX “button”) and Idle is mapped as release (as in releasing the very same button, you click on press, it changes to release and you move the throttle as if you were switching the engine off).
Another way to do it without using the AL is to recalibrate your throttle axis to make it include the portion before the detent and simply set the idle detent on the axis in the controllers menu inside Falcon, just like you would the AB detent.
Hope this is useful!