Under the flat lower frame, as far to the front as you can sensibly get it, ie: leaving a little room for error.
Avoid any lugs or other excresences which would unbalance the thing, so all four contact points (two per leg of the jack) should hit the frame at the same time.
Getting it as far forward as possible puts as much of the engine weight between the jack legs as possible so the bike will not want to tip forward.
Personally I use a pallet lifter, but the principle is the same.
PS: When you lift it off the kickstand, the handlebars will normally be turned to the left.
When you drop it, make sure the handlebars are turned to the left, and you have your foot on the kickstand. If they are turned to the right, the bike will fall to the right, which is not a good thing.
Please don't embarrass me by asking how I know :rofl:
PPS: Watch that the jack doesn't come up under the exhaust(s). Lifting the bike by the exhaust system will break something.