I don't think you can upload items for games you don't own, I would enable it if i could.

At the moment you can upload via inside the application - The workshop is built into the app directly, Workshop -> Share page -> Upload Item -> select items to upload, give a good title, and a description of what's inside, promote your stream, and give a realistic preview image so people know what to expect.

Thank you for your answer!
I appreciate you taking time to answer questions and fixing problems

I was also looking for this, it is nice to see that the developers do care for the community and answer like this :)