Standard solenoid uses 2 lines, Intake Manifold source and Wastegate actuator source, I can't remember which one is which, but with the engine running one of those lines will have vacuum while idling, that will be the intake manifold line (which runs to the back of the plenum near the clutch, but it's not an ideal intake manifold pressure source since it's already being bled, it's best to run another line for the intake manifold after the TB's) .
Pressure source is different from the intake manifold, pressure source can be T'd off at the stock boost map sensor (black box on the firewall near the brake master cylinder) which runs after the balance tube and the fuel pressure regulator (some people T off at the regulator as it will be slightly quicker to read pressure changes).
The nipple on the back is probably used to bled air and doesn't need to be connected to anything, the stock solenoid is a different type of solenoid as it blocks the line whereas aftermarket solenoids (mac valves) bleed air instead.