Use a screw anchor. There are a couple of different types.
Any ways to get a screw hook to stay in a ceiling? its one of those ceilings that crumble up into rubble instead of holding the screw. Im a bit worried the light will fall and cause a fire! im guessing "dont do it" is in order but maybe theres a way.
I found this old Q of yours from June...and I think Sudo the Dragon may have been talking about this also, but you can prolly use a molly toggle... here are photos.
You drill your hole into the ceiling, insert the molly toggle with the wings folded down and then the little wings are on springs and they spring out, holding the hook or whatever quite securely, quite a lot of weight!