Mattermost-Focalboard websocket error

Hiya. As of v5.36, Mattermost now includes Focalboard as a plugin at installation. I’d like to explore it, but am running into a websocket connection closed error whenever I try to load it. The error message links to Mattermost and provides two options for fixing it, one via NGINX and one via Apache. Problem is, of course, that while I’ve got an NGINX proxy in front of my Mattermost instance, it wasn’t set up as the standard Mattermost install steps suggest, and so I cannot find the sites-available/mattermost file they’d like me to edit. Any ideas about how to make this work?