I assume the pre made loom includes a relay, it MUST have a relay to be connected via the ignition.
Whip the tank off and you will see a group of connectors under the tank, on the left hand, left as you are sitting on the bike, there are two connector blocks, the lower one has a GREEN wire coming from the bottom, this is the switched live from the ignition, test it with a multimeter.
You can connect to this for a switched live.
DO NOT USE SCOTCHBLOCKS, they are an abomination, using wire stippers cut into the sleeving at two points 5mm or so apart, just cut into the sleeving don't cut the wire, then cut away this bit of sleeving leaving the wire intact but with 6mm of sleeving removed, then SOLDER the wire you wish to connect to the green wire and insulate.
This method will produce a joint that will not fail.
Within this group of connectors you may see, depending on the age of the bike, an empty connector, this is so the Meta 357 alarm can be simply plugged in but is NOT an ignition switched live.
HTH