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.

Thanks for the tips. Our functioning FullCalendar block on a production site broke after updating the module, which led me here. In case others run into the same problem:

If you try to set the "Click event" settings in a FullCalendar Block, and nothing happens (or it used to work but stopped working), go to the source of the FullCalendar feed. If that events feed is a Drupal view with a REST Export display, make sure to add the field "link to content" to the list of fields. Then, under "Format" go to the Settings link for the Show:Fields section. In the popup for "REST export: Row style options" find the "view_node" field and put 'url' as the alias (no quotes) so that the url will appear in the feed as url instead of view_node. Apply and Save. Then, in the FullCalendar Block instance that is displaying this feed, go to the "Click event" settings and choose one of the three options for what will happen when the event is clicked.

Thanks @potassiumchloride.

In terms how to debug the data fed into the calendar, you can use a develop tool, such as Chrome develop tool to inspect the json data return from the data source to see if there is URL link or any data you need for the calendar return in the json data.