When you opened to upload a new song, one of the boxes at the beginning asks about who the artist is and gives an option for me and others. Once you select that, you can then search for other artists registered on the site as your collaborator.

James, i think the "collab" sticker indicates it's a collab done, not a "open for collaboration". I dont think this sticker exists here yet...

Ahh, okay, I see now. Merci beaucoup 🙏