I Know that a chat is prohibited on scratch, with a white list too. I know that the only way to make a chat is to use predefined word because of an problematic of modération.
So my question it's : i do a chat (actually more a way to create and show web pages) where at each message the user needs my validation before posting content : is it autorized?

I don't think so, because
A. your authorization could be spoofed, and
B. Scratch won't trust anyone to moderate a project to the same standards as it moderates the site.