I thought the hangman noose was used for it, but if not then what is it used for?? And what's the difference when you hang with the slipknot vs with the hangman
You need a Noose Knot, not a Slip Knot. It's just a question of terminology, as the two can look identical, but it's an important distinction. If someone wants to create a noose and looks up how to tie a Slip Knot, it can lead to confusion, and they might end up tying the wrong knot! Here is a video that shows the difference:
About the question, as others have said, the Noose Knot slides more easily. A stopper knot should also be added when the Noose Knot is used as the Noose Knot can be unreliable in itself. Alternatively, the Poacher's Knot can be a good choice too. It slides well too, although not as easily as the Noose Knot. I wouldn't use the Hangman's Knot. There is too much friction, and the noose doesn't tighten well.