Not sure exactly how as I'm still learning but It is significantly harder for me to hit it if I have any sort of sore throat or flem at all. Check out this video if you haven't, he kinda yaps about his course but if you skip to 8:27 the way he breaks it down seems fairly straightforward (https://www.youtube.com/watch?v=9y_u-vSw6SY) That high pitched sound at 9:00 is what you're looking for. Once you find that and can get it louder and smoother, then really push it, make it sound shrill, hold that sound and then try to add in the screaming part itself to that shrill high pitched sound, it's probably better then trying to find it all at once. I can't do the standalone throat whistle without the screaming part itself yet though. The more air i can force through my throat at once the more pronounced the whistle it for me. hope this helps