Hello @EvelynW83 ,
Can you elaborate on what you mean by Join My Mailing List? Perhaps with a screenshot?
There are the new lead-generation landing pages you can find in My Campaigns. There's also the Inline and Pop-Up forms found in Sign Up Forms. It's also possible that you're using a legacy sign up landing page found under Basic Landing Pages in the Sign Up Forms screen. There's also the Facebook JMML plugin, is that what you're referencing?.