I have some past works I uploaded myself, but I don't know what the proper procedure is to flag these, either while submitting or after they have been uploaded. This feature is new to me and a precursory forum search didn't come up with much. Do I have to request an admin to link my account first? If so, then is this the right place to request it?

Thank you!

You simply need to get in touch with an admin and get your artist tag linked to your account. Then they'll automatically be marked as uploaded by the artist when you're the uploader of a post with your artist tag.