Hi, that looks good but in order for bulk filling, all you need to do is flip the rubber "seal" around. It just needs a way to be bulk filled from the front. https://www.toolsid.com/lincoln/grease-gun-filler-nipple-mpn-91108.html this is what needs to be at the front of the gun. You ram it into the delivery socket and pump it up. Just unsure wether the m18 comes with it, or even has a hole in it to put the nipple in. (I know, bad wording).
I swear I have seen those Milwaukees been bulk pump filled before