Not sure how that model works but it it has a float position, then use that and, if it doesn't stay there, pull it back quickly to neutral.
It should work with the engine off on most tractors but they might have wired it in differently on these new models for all I know.
When working the 3rd service I have h3 shuts grab and h4 opens grab bu find to release the pressure I have to go into the left hand dash screen and turn this function off so that it’s like an older joystick ( hold h3 and move joystick left or right to open/ shut grab) then stop tractor ,turn ignition back on hold h3 and wiggle joystick left and right. The bottom of the grab really needs to be on the ground so there’s no weight on it also. Clear as mud eh