Walk before you run.
Less than 0.0001% of airsofters will ever use this kind of setup at any time in their lives.
It is an adapter which allows you to run HPA to the bottom of a propane magazine. You can't "fill" the mag with HPA (and expect it to work).
It is usually used by people who want to test gas guns in their house without burning through expensive propane, and without the smell of propane, and the oil etc...
I have seen one person back in the day run an HPA tank to a quick change valve for gas pistols (change mag, then change adapter) for use in the cold... It was clunky, but cool to use.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.