Adding referer to an event

I wonder how I can categorize an event by referer (and/or campaign).

What I want to do: Know where the users who signed up for my newsletter came from (for example Instagram, YT, etc.).

The sign-up event fires onsubmit of the newsletter form.

What do I have to do, to view where the signups came from in matomo?

Hi @Splitter
I think the easiest way is the use of Custom report premium plugin: