How to add html code on the jot form to create your own custom text area?

You may use a "Text" element, but note that most of the HTML code is not required due to security reasons, please find more details about the form fields here: https://www.jotform.com/help/46-Quick-Overview-of-Form-Fields 

Also, you may simply add your text and format it with your own CSS code, this guide will help you adding the code to your form: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

I hope this helps.