Here is how you can add a new pick list field to a form that already exists:
- Open the form you'd like to add a pick list to.
- Click the edit (pencil) icon in the top right corner of the form.
- Click the "FIELD SETTINGS" tab.
- Hover over the new field you've added and click the pencil icon.
- Click the "Setup pick list" button.
- Add your list of options and click OK.
- Click 'SAVE'.
- Click DONE in the top left corner of the Edit Form Settings.