Why don't you ask the maker of the line lock? After all, they ought to know how they designed it.

Where you put it is up to you really.

If you use it in the rear brake line then you need to plumb/wire it so that it blocks pressure to the rear brakes. You will need to use your foot to put pressure on the front brakes. Safety issue here is that if you forget to release it you will not have rear brakes when you try to stop. Using this method can get tricky with a stick car.

If you use it on the front brakes you will need to plumb/wire it so that it holds pressure on the front brakes and you can use your foot for other purpose. Use the switch to release the brakes when you launch. If you forget to release the line lock it will be very obvious and you won't have a lack of brakes issue like you could have on the rears.

In either case, it needs to go after the warning light switch.