How to display the full name in Slack (when integrated with Matter Most)

[Smacktur]

Hi!
I'm asking for help to figure this out.

I need to display the full user name from Mattermost in Slack - how do I achieve this?
Right now Slack users can only see my MM login.

The UseUserName=true parameter does not help.

[unode]

We use:

[mattermost]
    [mattermost.server]
    RemoteNickFormat="<{NICK}> "
    PrefixMessagesWithNick=true

[slack]
    [slack.server]
    RemoteNickFormat="<{NICK}> "

But have the inverse problem.
Names from Mattermost are shown in Slack according to the value the user set on "Nickname" in their Mattermost user profile.
However, messages from Slack to Mattermost always show up with their username (or @handle) rather than the full name.