You can allow your users to add an option to single or multi-select fields. To enable this go to:
Form Fields
Click on the single or multi-select field
Advanced section
Turn on Allow users to add new options toggle
💡Note
When you are using dropdown for the selection field, you have to type in the new option and the option to add it will appear
Users can add new options while completing a multi-page form; the field does not need to be on the last page, and Save & Continue is not required for this. If you configure a limited list of allowed options for the field, users cannot add options outside that list.