Thank you very much fr your quick reply.
It helped me a lot.
The Article you linked helped a lot to understand the different parts of an element.
Also, with Google Chrome Developer Toolbar I was able to check the correct ID of the buttons I wanna add a popup.
I'm working on the page I wanna make the real changes : https://vikasayoga.com/yoga-teacher-training/universal-yoga/
and the buttons are working fine.
I have another button but it's on a Classic Card Element and for some reason I can't find the ID for that button.
So basically the idea it's have the same behavior on that button but because it's not a Button element but a Classic Card
If I change the ID like is this screenshot is not making any difference obviously:
Inspecting the element, I can find the button but not the ID of that particular button:
Thanks again for all the useful help