Or even just a prologue is fine. So long as the actual prologue/chapter is complete, then you can post it. But yeah, you can post what you have, since you do have a prologue and the first chapter complete. And then you can take the time to write the rest of the story and post when you're ready.
Haha, my section is Fanfiction and Writing. That's always what I reference to when I say that phrase. So I meant that you post in the main forum of Fanfiction and Writing, and not in the Writer's Lounge, since your chapter is ready to be posted.