Just a call out. I figured out the issue for my instance of the "access_denied", not sure if it was the same for Chen. I was not specifying localhost:8080 as my redirect.
I am in the same boat as Hanlin Chen. I can get the the auth code via step #3, but no matter what I do I get "access_denied" for step #4. I also am an admin of both Zendesk chat and support. What c...