You would put it where you put the writing, story and aging tags currently in your story. When in the edit screen, it's the little box down by the "Post" button.

In case you didn't know, there is a "five tag" bug in the current streemit software, so don't use five or more tags, but if you want to be read first by the fiction-trail, put fiction as the first tag in that list of four that you have.

Anytime you use the #hashtag in the post, or put it as the first one in that box, that tag is used as your first tag and that decides where you story goes in the file system. For instance, your link above has "writing" in the URL because you put writing as your first tag. That makes searching for the post easier.