Technically and semantically it would be a “submission”, but that might get confusing if you adopt the “sub-Prismo”/“sub” notation. You could then hypothetically “sub(scribe) to a sub(prismo) to see sub(missions)”.
As far as “post” vs “story”, the first one emphasizes the act of user-submitted content, and the second emphasizes a narrative element. I’m not sure “story” sounds appropriate unless a given group exists to post links to, well, actual stories. So I think “post” might be more appropriate out of those two; see also: reddit “self-post” or “cross-post” terminology.