A kill switch for a car
I guess you want a hold circuit. It's common to put a relay to switch voltage to its own coil. An external switch is pressed to make the relay pull when the system has been unpowered a while. Only a momentary press is needed.
BTW Beware inserting hobbyist grade parts to a vehicle. It's begging troubles (NOTE1). A car is very hostile environment for all electric circuits.
NOTE1: Think a day you are crossing a highway. A truck is coming, but you have plenty of time to drive out of its way. Or so you thought. The kill switch happens to work literally just at this moment. If you like to think this do it beforehand. Later it will be impossible.