Lesson 2-1-21 - High A# / Bb
High A sharp (same note as High B flat) is one of the nine half-holed notes in the standard 25 notes for a Key of D Whistle.
It's made by covering the top hole completely, then half-covering the second hole, leaving the last four hole uncovered, and blowing quite hard into the mouthpiece
I find that bending the left middle finger and using the tip of the left middle finger (as opposed to the flat of the third left distal phalanx) works best for what is probably the most challenging note, from both a fingering and a breath control perspective.
As with all the half-holed notes, I'd recommend learning the full-holed notes first before trying them. There are fantastic whistle players out there who cannot play these notes with any degree of proficiency, and they still manage to play fine music.