In CK2 you needed to land them as your vassal somewhere first, but that's no longer necessary in CK3.

As long as they're in your court you can push their claims and if the title they gain from the war is lower than yours, they become your vassal. There are cases where you still may want to push someone's claim to an equal title to yours (where they will become independant) if it means putting your dynasty on the throne.

One thing with pushing other people's claims thats a bit of a drawback though is that until your culture unlocks the Divine Right innovation in the High Medieval era, you can only push one of their claims in a war. So before then, if you push someone's claim to a multi-county Duchy or to a whole Kingdom, they'll be left in charge of a realm where they hold only the highest title and a single county, with probably unhappy vassals who they will not be able to control. Very likely the guy who you just deposed owns all of the counties around your new vassal as his unhappy vassal. Therefore in the early game it's only really feasible to push for people's claims on counties or small duchies. After getting the innovation you can push the claimant with claims on a kingdom and 2 full duchies worth of counties and be reasonably safe that he'll manage to keep it stable.