Unless you are fabbing a setup yourself, do what the instructions say to do.

You want vacuum in the booster BEFORE you touch the brakes, so hooking it up to the brake lights is a bad idea.

Just power it by a keyed source and use a vacuum switch to turn it on and off on the ground side (might need to go thru a relay to protect the switch from too much current).

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.