The mc cable has connectors intended to enter boxes via a 7/8" hole. The connectors are considered 1/2" the hole is 7/8" All connections are to be in listed boxes. In the case of post lights I have often seen the connections made within the brick post , but in that case the wires often corrode. They do make post light boxes, you may have to contact an electrical supply company to find them. You will need to remove the armor from at least 6" of the wrapped conductors, slide a smal bushing around the conductors between the armor and the conductor. Then you slide the mc box connector around the conductors down over the armor. Tighten the hold down screw. Pass the free conductors through the 7/8" hole in the light fixture and press the connector into the box.