Has anyone used that Gitlab/Mattermost document and has gotten the system to work without any additional changes that have not been mentioned there? I think I'm incompatible with their documentation 
I'm probably missing something quite obvious. I have gitlab omnibus running inside of a virtual machine on a Windows Server in a network that I don't have admin access to.
I wan't to activate Mattermost.
Following the documentation i put down "http://mattermost.gitlab-srv" as the mattermost_external_url (gitlab itself is reachable at "http://gitlab-srv").
Now, if I take the documentation at face-value, that should be it (spoiler alert: it isn't).
I probably need to add the subdomain somewhere but I'm not quite sure how to do that (can I do that on the virtual machine im running or do I have to add that in our networks DNS server?), and where to point it to get this running.