The existing one is from showHeaderCopyrightWarning in EditPage.php, which shows the editpage-head-copy-warn message.

I've previously raised a similar thing here. @Whatamidoing-WMF pointed out that there's already a notice of sorts for new editors trying out VisualEditor. We made some adjustments to the wording, stored here (see the talk page), so I'm not sure there's all that much else to do.

As a suggested avenue for future development, it'd be nice if tools like Twinkle/RedWarn linked to the how-to-add-citations page when reverted unsourced material added by new editors.

In T300942#8151596, @Sdkb wrote:

We made some adjustments to the wording, stored here (see the talk page), so I'm not sure there's all that much else to do.

The message cited in task description additionally concerns copyrights, linking to Wikipedia:Copyright violations page in enwiki, while this Cite tool popup text only urges users to cite sources.

New/careless users often copy and paste, or alter the source material only a little, and hence violate copyrights (no matter whether they cite sources or not). WikiEditor displays associated warning message at the top of WikiEditor and always (not only in one-time pop-up). I don't think there is less reason to display this important message prominently in VisualEditor.

I'm not sure where in VisualEditor this message should be displayed, though. In "Save your changes" dialog above edit summary field perhaps?

In T300942#9597055, @Pikne wrote:

New/careless users often copy and paste, or alter the source material only a little, and hence violate copyrights (no matter whether they cite sources or not).

Aside: do you have links handy to 2-3 edits that show the above happening?

...I ask the above in an effort to add some evidence/context to the following row within mw:Edit check/Ideas: "Prompt people who are pasting text into an article to confirm the content is not at risk of creating a copyright violation."

Aside: do you have links handy to 2-3 edits that show the above happening?

In enwiki you can look for examples in maintenance categories like this, or look in deletion log for speedy deletion/redaction reasons G12 and RD1. Recent log entries refer to e.g. this page and this page where copyright violations were redacted today.