This is the dump truck mission accessory for the M.A.X. Truck Modular Toy truck. The base vehicle may be found here: https://www.thingiverse.com/thing:3214824
Snap the dumper beam to the dumper rail mount before attaching the dumper beam to the dumper with screws.
The dumper is held to the dumper beam with four M3 x 6 screws. The rear most screws are a little tricky to install, but it's definitely achievable. Place the screws in the dumper beam first, then screw them into the bottom of the dumper. You'll need to do this with the screw driver / Allen wrench at an angle, as the hinge for the dump assembly is intentionally limited in motion.