Don't mess with the safety valve (that's the screw) on the pb cylinder.
Attach the adapter securely to the reg, then screw the whole thing onto the pb cylinder. It will hiss briefly as you tighten it down. Just make sure your solenoid is off and your working pressure is at zero when you attach it. For as long as the adapter is attached to the pb cylinder, the cylinder will be "open". That depressed pin is the equivalent of the on/off valve at the top of a standard industrial cylinder.
I'd still recommend a 5, 10, or 20# cylinder over a pb cylinder any day, but I assume you have a reason for it.