### /etc/kopano/webapp/config-mattermost.php
### Set up Mattermost URL
define('PLUGIN_MATTERMOST_URL', 'https://mattermost.<Domain>');
### Headers for Content Security, sample code in the vhost files.
[...]
### Optional headers to access Mattermost from the Kopano WebApp
Header set X-Frame-Options "ALLOW-FROM https://kopano.<Domain>/webapp/"
Header edit Content-Security-Policy ^(.*)$ "$1;frame-ancestors 'self' https://kopano.<Domain>/webapp/"
Header unset X-Frame-Options
Header unset Content-Security-Policy