I usually write something like this:
"Thank you for considering my short story, TITLE (xxx words). My short fiction has previously appeared in X MAGAZINE and Y MAGAZINE. I look forward to hearing from you."
Yes this. I always use that space for my cover letter info. I just copy and paste the cover letter directly, actually, 90 percent of the time.
Like Libby said it's just a really brief and professional, "Hullo, this is my work and wordcount. I've published here. Thanks again." Only a bit more professional but you get the idea. There are plenty of examples of cover letters online.
Just be careful and read all instructions to make sure they don't want something specific in that spot.
EDIT: are you talking about short fiction? Because that's where I usually find those boxes--online short story/poetry publishers. I suppose if you were doing a serial novel or there was a journal taking full length books it's possible. Not sure about a full-on query for a longer work. Will leave that for others to answer.