Problem to enable Mattermost on gitlab

Hi,

I've actually the last version of Gitlab, using a no-embed nginx webserver.
I'm trying to activate Mattermost on my Gitlab but no success.

Here is my gitlab.rb :

external_url "http://gitlab.xxxx.fr"
nginx['enable'] = false
web_server['external_users'] = ['www-data']
mattermost_external_url 'http://chat.xxxxx.fr'
mattermost['enable'] = true
mattermost_nginx['enable'] = false

but when i'm trying to access to chat.xxxx.fr i'm going to xxxx.fr.
I think the vhost for mattermost is missing, but i've no idea of the "good" vhost to make this work.

Any help to get mattermost on gitlab working ? Any help appreciated.

(sorry for this newbish question) ...

Thanks a lot.