Proxy settings for outgoing connections, e.g. push-proxy

Yes I am aware of that
I want to use the default push.mattermost.com
BUT I want the connection to go through a proxy, as my mattermost-application server does not have direct access to the internet. All communication to the internet must go through our company proxy.
Right now mattermost tries to contact push.mattermost.com directly which will not work as the firewall will block it.
Mattermost application must be aware of that it need to contact push.mattermost.com via our outgoing proxy.