There should be a line that is capped off, it comes from the same port that the fuel pressure regulator gets it's vacuum on the drivers side below the supercharger. You need a source below the supercharge that way you will see both vacuum and boost.
Here is a Vacuum diagram although it is for a lightning, as i said in a Harley truck the line usually is caped off.
http://www.lightningrodder.com/forum...1&d=1508700694
By the way, you can poke a hole in that big rubber grommet to the right of your red circle to run the line inside.