Help, I can't figure this out because Fandom is mysterious to me

For fandom_stocking, I started a drabble but it turned into something pretty lengthy. (5k+, probably.) Do I post the whole thing as a comment? Can I post it to FF/AO3 in secret, somehow, and leave a link? I could really use the chapter-break option of the archives, plus not having to go through and fix all the italics, (that I over-rely on italics in writing is a different story) even aside from the fact that posting 5k as a comment feels odd.