I'm a divi user as well. Right now, the modal form only pops up when you choose a setting of one of these 3 things:
- Exit intent - the user moves to exit the page
- Scroll percentage (you can customize), or
- Timing (you can customize)
There's no way to set a modal form to a button in Divi. Instead, you could:
- Redirect users via a divi button to a convertkit landing page
- Keep a CK form on the page at all times (sidebars are great for blog pages)
- Choose one of the 3 existing modal options.
If it's a feature you'd like to see in the future, you can add it to the #FeatureRequests channel so the dev team can see it. :) Hope this helps!