Etiquette for writing a story in someone else's "universe"?

I notice some stories that are in a "universe" say "authorized" or "permission" (from the original creator of the universe) and others don't.

I'm writing a story in one of the universes (the S.P.A. Universe), which I might consider publishing here if I ever think it's any good, but want to be respectful to the creator of the universe. What is considered the polite way to handle it, on the off chance it turns into a story I think is worth posting?