Hey Jen 
You can do this by setting the state on the resuable header. For example, on the page with the button, you can create a workflow which “When favorite button is clicked and current user isn’t logged in”, element actions --> set state --> element: header, custom state: popup (for example, type: yes/no), value: yes.
Then inside the resuable element header, create a “Do when condition is true (every time)” event which is “When this header’s popup is yes” --> Show Login/Popup.
Then create a second event inside the reusable header which is “When Login Popup is closed” --> set the popup custom state back to “no”. I’m pretty sure this should work. If not, you can go to the Settings tab and set the app to be “Everyone can view”, then paste in the editor link. Or feel free to PM me if you prefer.
*My mistake, I think the creation of the custom state has to happen first inside the reusable element, not on the page.