Customize Fields on Zoho Campaigns Sign Up Form

Customize Fields on Zoho Campaigns Sign Up Form

Hi, I'd like to customize the buttons on the sign up form. This form will allow users to sign up for a newsletter on my website. I am using a Theme via Zoho Campaigns, however the tool only allow for formatting changes of the form. I'd like to add a new text field to the form. How do I do this in Zoho Campaigns?