Whereever you like. From a quick recce it appears that streetlamps do not appear on the map, but they do have ID numbers. So find out the ID of your lamp and put
object 1234 switchLight "On"
in your init.sqs as you said. Or in On Activation field of a trigger, if it happens during the mission. Or a waypoint.
All code can go in script files, triggers, waypoints, init fields ... whereever is most convenient for you.
Edit: just because I didn't say "syntax not guaranteed" doesn't mean it is. NEVER trust my syntax, always check it in the command ref.