Custom scales are defined in the same spirit as custom chords. You only define a sequence of intervals and not a sequence of notes. The app will then generate when needed the 12 corresponding scales (one for each tonic).
In order to create a custom scale, go to the chord selection screen, select “Custom”, and then tap the “Add” button
Your custom scales will be used as any other scale in the app. They will appear in reverse searches or when harmonizing chords, and you will be able to compute their fingerings for example.