How do I make a drop-down field fillable by applicant

I've got a drop-down menu with pre-defined zip codes.  If the applicant chooses a town or city, the zip code automatically is filled out, but occasionally I don't have the zip code number, so I want the applicant to be able to fill in the zip code themselves.  How do I make that field fillable.

To achieve your requirements, you can use the User Contributed Dropdown widget instead of the regular drop-down field.

I hope this helps. If you have other questions or concerns, please do not hesitate to let us know.