Not sure this helps or is more confusing:
Using Restify and NodeJS at work with its Mattermost server (testing at work, not installed by me), I could access the user_name (and similar) parameters via req.body.user_name when using POST.
When using GET that did expectedly not work. However neither did req.query.user_name.
I guess I’ll have to learn how to read the source code of Mattermost 