In your thread title, it says the knot is what's getting under your voicebox? Or just the rope? You'll want the knot directly behind the back of the neck, center top, almost at back of the bottom of the head kind of.
When I've practiced the rope didn't go under my voice box, but it's also uncomfortable to swallow, and so I try not to.
When I do it, it's up like against where my jaw and neck meet and I hold it taught before lowering so that it doesn't slip down as I go to tighten it/lower myself
Also, if you're just practicing, you don't want to use a constricting knot, more like a static loop so it doesn't constrict and incapacitate/kill you before you're ready
Also also, I hope this goes without saying, but don't practice with your rope secured to an achor point. I hold it in my hand, I don't twist it around my hand to help the grip bc that can catch and make it harder to let go once you start passing out