Adding custom class to a button

Hello,

up front: I did already search the forum and read through the suggested solutions but couldn’t solve it.

We have developed a plugin (my programmers) that is embedded on the site via shortcode.
The plugin creates a button that opens up a custom contact formular.

Now I want to add that “trigger” to a custom “standard” button in enfold.
My programmers told me:
” ‘cbtn’ is the class name that you can given to any button and it will perform like the contact button
<…….class=’cbtn’………………/>”

How can I add that in the button properties dialog?

Thanks