The F sharp dominant flat ninth chord is a 5-note chord consisting of the notes F#, A#, C#, E and G.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as F#7b9.
Interactive piano diagram for the F sharp dominant flat ninth chord
Chords that are a subset of F#7b9. These contain less notes but all of them are included in F#7b9.