As a contributor, add a "AC Forms" Gutenberg block to a post dn put the following payload in the "Additional CSS class(es)" advanced settings of the block:

" onmouseover="alert(/XSS/)" style="background:red;width:100px;height:100px;"

The XSS will be triggered when viewing/previewing the post and moving the mouse over the red block

Note: There is no need to connect Active Campaign to confirm the exploit.