Stuck?
The chords, notes and shapes that fit your song.
A chord on its own is a shape. A chord in a key is a decision.
See what belongs to the key, beyond the chords already in your song.
A box for the lead. A root for the bass. A note for the singer. The same chord, answered for each instrument.
Borrow a chord from outside the key and it won't pretend to number it. Harmony notes come from the chord, not your melody — and they say so.
Worked out on your phone. Nothing leaves it, and no signal is needed.