If you have admin permissions on your Codacy self-hosted instance, you can set up or configure your GitLab.com integration by going to the Admin panel on the top navigation bar.
When you open the GitLab panel, if you haven't created an application yet, this is the first step you need to take. Just follow the link.
Creating a GitLab Application
Follow the link to create an application on Gitlab.com
You need to ensure the "api", "read_user" and "read_repository" scopes are checked.
You'll also need to add the following 'Redirect URL's. Make sure to update your protocol to use either HTTP or HTTPS and your domain name as well. Keep in mind this field is case sensitive.
After this, copy the "application ID" and "secret" and paste those to the GitLab integration panel on Codacy.
Hit "Test & Save" and you should be good to go! 👍