I was just able to add one to a story. Try this:

Write your Title and subtitle.

Put your cursor in front of the Title and hit return.

Type in your Kicker in the line above the Title, and then highlight it and choose the smaller T (that you'd normally use to create a subtitle).

If you can't get it to work, just make a note somewhere in the beginning of the story that it's for the erotica challenge and I'll add it for you.