Took me a while to reply. A#/F# is an F# chord with a A# bass note. Look up C shaped barre chords. F# root is on 5th string 9th fret. A# is 6th string 6th fret. Thumb over is Hendrix style.
I just used it as an example because I’m learning a song with that chord now and can’t play it that way, I had to use alternate fingering and pluck the strings.