Publish Fortnite Island?

Does jmd know how to do that? So now? I find nothing right on the Internet

You need a creator code, otherwise it won't work.
You can find more information here: https://www.epicgames.com/...ew?lang=de

To get an island code, you first have to do the following:

You must have your own Support a Creator Code.
To do this, you need to have 1000 subscriptions on any social media platform.
You must be able to receive the income from the Support a Creator Code in any form (email, post, etc.) from Epic Games

Then you can request an island code from Epic Games.