That looks like the right thing..
You would put in the key, and give it a turn(making the contact neccessary to start power). Then turn it back, and remove the key. Power would stay on. When its time to shut down, you would repeat. Except this time the contact would kill the power
Ive got one of these keylock switches for my 'free credit' mode. Ive got the reject buttons on my door wired to give credits when the key is turned 'on', but when it's 'off' it breaks the ground to those 2 switches and they stop working.
I pulled mine out of an olllllllllllld pc, back when key locks were pretty common