you don't HAVE to put anything. If you don't give Nef any information, she'll just decide everything for your breed on her own.
Otherwise, you can request the breed type and you can give her an idea of like what kind of height/build range you'd like, and maybe like strongest stat. But don't bother being too specific, because you will not get much of a say in stats and build and such. I believe Arqy mentioned something about her minishetlands being colored and did get some painted foundies (and an albino

). Just don't come off too demanding or she might give you the exact opposite of what you want
You can include a brief description/bio and she'll likely put it in the description spot on the breed pop-up (you know when you click a horses breed and see the breed standards)