Add custom chords to GarageBand for iPhone
You can add your own custom chords to a song. Custom chords can include extensions (added notes) and alternative bass notes. When you add a custom chord, it is available to all instruments played with chord strips in the current song.
Add a custom chord
Tap the Settings button in the control bar, then tap Edit Chords.
On iPhone SE, iPhone 8, iPhone 7 or iPhone 6s, tap the Settings button in the control bar, tap Song Settings, then tap Edit Chords.
Tap the chord strip you want to use for the custom chord.
Swipe the Chord wheels to set the chord root and chord quality, and to add an extension (added note). Swipe the Bass wheel if you want to add an alternative bass note.
When you finish, tap Done to return to the song.