How to use FG ground on a meanwell buck DC\DC power supply
The FG pin is to ground the emi filter to a low impedance ground, in the manual this would be earth. Since that isn't possible on a forklift (because you'd probably want to move it) then connect it to the chassis. By connecting it to the chassis EMI from the cables or supply can return back to the supply through the FG pin instead of radiate.