Adding a custom drop down/popup text tab to the player

Hello.  I am editing/updating an existing Storyline course from a client (I am using Storyline 360; the original course was made back in 2016).   They have a References tab in the player, added as a custom tab I've assumed.  When clicked it slides down a popup box that looks just like the box that comes up when you click the default Resources tab.  It just contains a text list of reference citations.  

I cannot figure out how to edit or insert that text.  In the Player settings the preview shows the box slide down but it has no content; when I preview the full course the text is all there.  Where is  it coming from?  is it somehow javascript that includes the content?  if so how do I access it?  the Edit button is grayed out when I have the tab chosen in the list.

Is it possible this was a feature in an earlier version of Storyline that is no longer available?  Thanks for any help you can give.