Yuriy, why don't you create a form directly in OneBox, Formy's supplement.
Th form through the iframe, which you write in the WordPress HTML block, add the change that you have created.
If you create such a form, then you will immediately begin to transfer requests from the site to the system.
The axis of the butt is such a form on my website
https://icoloronebox.org.ua/products/license/Screen 1182