Yep, it's designed to use a stereo jack to act as the power switching for the preamp. Inserting a standard mono plug into the stereo jack bridges the ground and SW via the plug's sleeve, which completes the power circuit turning on the preamp.

If you really wanted to you could use another toggle switch or push/pull pot to act as your power switch, but a stereo jack is the easiest and more standard way of doing it.