Edits autocomplete.json - open it from Project Files → autocomplete.json. Doesn't need to exist ahead of time; opening it with no file yet just starts empty, and the file is created the first time you save.

• Add, rename, and reorder keys - each key is a named list of suggested values (e.g. a make key listing car makes).
• Each key has its own editable list of values - add, edit, remove, reorder.

Wire a key up to a field in the Form Editor by setting that field's Autocomplete Key to match. See Fill Out a Form for what this looks like at fill-out time - suggestions only ever come from what's curated here, never from what someone previously typed into the form.