Mattermost integration from Gitlab - still use the Slack service template?

I am not sure what service template to enter to make Gitlab post messages to Mattermost. Still use Slack? How do I determine what hook URL to use? My mattermost is on the same physical machine on a different host name (virtualhost).

The mattermost documentation is completely unclear. Posting this here so anybody who hasn’t found the mattermost forums will be able to find any discussion that occurred over there.

There are some details and setup over there.

I’m a bit dismayed that there’s no Slack test button on the Service Templates or the Project Settings “Services” tab in gitlab. Nevermind, I will just use POSTMAN, which works, but Gitlab itself is not actually sending me anything yet, and I don’t know why.