The power can come from anywhere,it just depends on whether you want it to work with the key on or off.
I agree that it would be better not to have it switched through the ignition so you could use it without the engine running.
You should be able to test the posts on your headlight switch with a voltage tester, or a test light.
You'll want one that has voltage present even when the key is off.
Turn the key to the off position.
Find the post that still has voltage.(there may be more than one)
Disconnect the negative battery cable.
Run a wire from the post that was hot to a fuse, then to the spotlight.
Reconnect the negative cable, turn on your spotlight.
