The starter solenoid wire will only be hot in with the ignition switch in the crank position. Once you let off the switch and it returns to the run position that circuit will be dead.
Copied this from their website:
Q: Where do I connect the hot wire to make sure I get 12 volts to my DUI?
A: Your hot wire for the DUI must be at least 12 gauge and it can be connected to the starter solenoid or a fuse. The solenoid is usually
the easiest place to connect your hot wire to but make sure you use the terminal that is hot only when the key switch is in the start and
run position. Same with using a fuse, just make sure the voltage is present only in the start and run position