As a contributor, add a "WL : FAQ" Gutenberg block to a post, and put the following payload in the "Additional CSS class(es)" advance settings of the block:

" onmouseover="alert(/XSS/)" style="background:red;"

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

Note: A Template Builder needs to be present (one can be created by an admin at /wp-admin/edit.php?post_type=woolentor-template)