Are you trying to access an element on your page from within a reusable element? Have I got it right?
In general, a custom event is triggered on the reusable element from the page, but if the answer to my first question is yes, you have to use this plugin.
What I mean is that if you want to show the same element (pop-up, group focus etc.) on more than one page, you put this element inside the reusable element and create a custom event inside the reusable element.
Inside the reusable element, workflow tab:
Custom Event > Show Pop-Up menu
Then you can use the action “trigger a custom event from reusable element (header)” from any page where you put the reusable element.
Inside the page, workflow tab
A button is clicked > Trigger a custom event from reusable element.
Also, these two free apps will help you. Actually, these are templates, but they were created for learning and reviewing examples, not for creating apps.
For actions: Custom Events | Actions | Bible
For operators: https://explore-bubble.bubbleapps.io/
If there is anything you want me to explain, please feel free to ask.