How to confirm yourself as the author of your work?

Some of my works are on other sites in free access, without my consent. After I sent a complaint to these sites, I was asked to confirm my authorship documentarily. But how to do that? On енвато I have not found, where it is possible to download the document confirming, that I the author of the work?

There’s no document that you can Dow load to confirm ownership as only you can confirm you made it… just uploading an item to Envato doesn’t automatically mean you own it, so it would be risky for Envato to confirm anything like that.

A link to the item with the date of upload might help though… it’s highly unlikely you’d upload the item somewhere before somebody else did, if you weren’t the owner.

Thank you! You helped me a lot!