Is that permitted? And if so, how do I state the permission and source?

If not, what should the artist do to allow it? Should they make the post public? Or maybe upload it themselves?

In this case, the artists have not public nsfw galleries, and they want someone to upload something that is on their patreon.

The artist themselves can post their own paysite content, and if you can demonstrate permission from the artist to post said content you can post it too.

snpthecat said:
The artist themselves can post their own paysite content, and if you can demonstrate permission from the artist to post said content you can post it too.

okay, thanks!
And for the second case, how can someone demonstrate permission when uploading the post?

ruribec said:
okay, thanks!
And for the second case, how can someone demonstrate permission when uploading the post?

A link to a screenshot of the permission being given linked in a comment (hidden if desired) or the description will work. Or you can contact a Janitor or Admin with the proof and we can make a note of it.