Where to find Mattermost after installing GitLab omnibus?

Sorry for asking a very stupid question: I have modified gitlab.rb to include

external_url 'http://casgitlab.casalelug.ch
mattermost[‘enable’] = true
mattermost_external_url ‘http://192.168.0.213

but I still do not understand how to access mattermost. Which url should I call?