For security reasons, our forms only support the pre-defined fields you see in the editor. So, in order to customize a form with your own fields, you will need to create the form on an external site and paste the embed code into a new section on your site. Let me know if you need instructions on how to embed a form code in a new section.
There are many free form builders available. Here is one that works well:
https://embedsocial.com/forms/
Let us know if you have any other questions or need further instruction.