We are trying to use the form composer to create an A/B test that adds a link to the end of a div on every landing page. I can use the visual composer to add it to a given page, but this needs to be added to multiple pages, so I'm thinking it needs to be done via a form. I've added an mbox via DTM around the given element, but when I try to add the link it messes up the CSS (or completely replaces the element I'm matching in DTM). Any ideas on a better way to do this?
Have you considered to use multi URL setting in the activity configuration as an alternative for the mbox implementation? The setting can be found in VEC by clicking on gear icon > Page Delivery (add as many URL masks as you need to cover all landing pages).
Alternatively, consider implementing an mbox parameter on the landing pages you want to target the activity to. Then create an Audience to include the pages where that parameter exists or has a certain value. The activity URL then should be set to any (e.g just target the activity to the site domain).