F addb9/F#
The F addb9 chord consists of the F (Perfect unison), A (Major third), C (Perfect fifth) and F# (Minor ninth) notes, with F# as the bass note.

How to play the F addb9/F# chord
- Play F# as the lowest bass note in the left hand
- Find F (Root) on the piano keyboard
- Add A (Major third) — 4 semitones above F
- Add C (Perfect fifth) — 7 semitones above F
- Add F# (Minor ninth) — 13 semitones above F
- Press all 5 notes simultaneously to sound the F addb9/F# chord