Waiting for my pin2dmd to arrive. Still confused how to connect it to 12V despite several posts describing this. Maybe I'm just too dumb

I want to take the safe way and connect it to an external power supply. My question is how to connect the external power supply to power on when machine is powered on?

My guess is to buy a regular 12V 5A DC power supply and cut both ends on it (i.e. remove the wall plug and the other end).
Then connect the output side of the power supply to the dmd and the input side (where the wall plug was) to J605 (the violet wires that gives 100VAC).

Can somebody plz confirm that I got this right? Is it correct that connecting to J605 is the same as connecting to the transformator, so the dmd will not be using the HV section on the dmd driver board?

Is there a better way to connect it to an external power supply?

The game is The Flintstones.