That fitting is generally used to de-gas the cylinder or else use it to hook up to a DIN style valve. The easiest way to connect your HW to an external air source is to just use the air fill probe included with the gun, which is then attached to the external air source.
That's supposed to screw into your DIN300 filler tank.
You need a FEMALE DIN300-Foster adapter to make it work for your tank if it's not DIN.
You need a FEMALE DIN version of this or a threaded sleeve to use it with your brass filler. A machinist can make one. Double female DIN adapter both screws into.
No, not tethering it. I have 2 HW100 rifles, and occasionally I will fill one cylinder while off the rifle. Obviously, I can do that by using the standard fill probe, just as I would when the cylinder is on the rifle. But the pictured fitting is designed to attach to the cylinder, and fill from the tank. But, that fitting does not fit my tank, the O ring position is wrong, and it doesn't screw in deep enough. I have a couple of adapters as others have shown, they will not work this fitting.
The reason these adapters are not working for me is due to the yoke design on my 4500 psi tank. It is deeper than "standard", so I need a longer insert, if one exists. But I'm not going to spend any more time on it. As I mentioned, I can always use the probe, even with the cylinder off the rifle, so it's no big deal. But thanks for all the responses.