Silly question about hooking up hot water outside

What you saw is probably the high pressure release valve. Usually they don't have a threaded spigot but you may be able to jerry-rig something to attach to the spigot and then open the valve by flipping up the lever that is on top of the valve to get hot water to flow.

Your best bet would be to have a plumber or handyman tie into the hot water system and plumb a spigot into an outside compartment for your use.