How to add a dropdown field in the ticket form and make it available for customer to edit?

In your ticket form, you would be able to add a dropdown field and make it available for the customers who are raising queries with you. Please navigate to Admin > Workflows > Ticket Fields -> choose this field which is a list icon with an arrow pointing downwards found in the "Drag and drop down" toolbar.

In order to make it available to the customers to edit, please click the field to navigate to the properties. Once there, check all the relevant boxes under agent and customer which includes the "customer can edit" option.