How to make popup/click work?

Here is the document from Fullcalendar V5,

https://fullcalendar.io/docs/v5/events-json-feed

https://fullcalendar.io/docs/v5/eventClick

The key point from the documents above is that

Normally, if the Event Object has its url property set...

Ah, thank you. What I was missing was simply adding the event URL to the json feed and naming it "url"

I didn't see this in the module docs. If it's not there then this may be helpful to add.