What is true of it
C♯add9 is C♯, E♯, G♯ and D♯ — 1, 3, 5 and 9 counted from C♯.
On an instrument that cannot reach all 4, the 5 goes first, in that order.
The easiest of the 1 fingering below is 6546: 4 fingers, held at the 4th fret.
None of them uses an open string, so this chord moves up the neck unchanged — the same shape 1 fret higher is the chord a semitone above.
Where its notes are on the neck
The brass dots are C♯, the root. Everything else is a chord tone with its degree printed in it.
With a different note underneath
C♯add9/G♯
1314
C♯add9/D♯
8898
An inversion is the same chord with a different note lowest. The name changes to a slash name because the bass is what a listener hears as the foundation.