Refer to CowboyBob's post. This is basically what I was going to answer.
But, congrats on your NSH! They are super fun and awesome (although I am quite biased in this opinion ;) ). They are also extremely smart and catch on to things super fast, so teaching her this shouldn't take too long, but it's a matter of being consistent with your aids, otherwise she will find a way to evade you, as I assume she's already doing.
Another good idea is also training a head up cue so that you can cue her to lower her head by finger pressure on the poll, but she doesn't raise her head back up until you ask her for it. With my horse it's pushing on the bottom middle of his neck to come back up.
Being consistent with it you will probably be able to get her to the point of just hovering your fingers over her poll for her to drop the head.
Release is so important!
Happy training, good luck!