One solution for this is to have an "Other" option in your field and a new field (text element) where your user can type and will hold a value for the "Other" option. The new field can be hidden at first and will just show once "Other" is selected using Rules.
If you want the new field's value to be copied in the first field upon submission, you can use trigger for that. Here is a documentation: https://howto.caspio.com/tables-and-views/triggered-actions/
