Hello @viraltechmarket , Welcome to forums!
From what I understand, and based on what’s stated on the official page and what I’ve researched, you’re not required to have created an island beforehand.
The site doesn’t specify a particular order , it simply mentions that, to be able to publish, you first need to have signed up for the program (“signed up for the Fortnite Island Creator Program”).
I’m leaving the official documentation where everything is explained in more detail.
Fortnite Island Creator Rules
https://www.fortnite.com/news/fortnite-island-creator-rules?lang=en-US
And if you have any questions, feel free to ask again in the forum.
Hope it helps!