[grannyknot]

There are a couple of ways to do it,
find the power wire for the lighter, cut it and solder a length of wire to each end and attach a simple toggle switch to those ends and hide the switch somewhere that's it is not going to be seen. Switch it on when you want power and off when you don't.

The other way would be cut that same power wire going to the lighter, tape off the power side so it won't short to ground then tap into another circuit of with the same amperage rating (fuse rating) but is only powered up by the ignition switch.

When looking for a switched power wire to tap into, look for one that is at least 14ga or thicker.
Any particular reason why you want that circuit switched off?