JavaScript can be added to an onMessage page by using the embed widget.
  1. Navigate to onMessage > Website
  2. Choose a page from the menu
  3. Select Edit Page Content from the wrench icon drop down to the right of the page name
  4. From the left task bar, drag in the Embed Widget onto the web page
  5. Select the gear icon on the widget 
  6. Add the JavaScript
  7. Select Save
User-added image