1) Under "Products" Menu on the left side of the builder

2) Select "Products" 

3) Double click on the product you want to add HTML code to

4) Scroll down to "Options"

5) Click on the Dropdown arrow on the top right corner of the "Options" window.

6) Toggle the "Add to Cart" button from "Show" to "Hide'

7) Make sure you publish your changes so they appear on your live website.