I install MC 28.0.80 64-bit on Win10 pro and Win11 pro.
JRiver MC options:
Media Network > "Use Media Network to share this library and enable DLNA" (Access Key: xxxx)
Startup > Windows Startup > "Run on Windows Startup: Media Server" (as described by OP).
I find after PC login:
Task Manager > Processes tab > Background Processes:
Media Center Service > JRiver Media Center Service
Media Center 28
After opening the desktop MC GUI, Media Center 28 moves from Background Processes to Apps.
Task Manager > Services tab:
Media Center 28 Service (Description: JRiver Media Center 28 Service)
This PC > RMB Manage > Services and Applications > Services:
JRiver Media Center 28 Service (set Startup Type: Automatic)
Prior to login I can connect to JRiver Media Center 28 Service and use it as a music server, for example with BubbleUPnP App on Android, JRemote2 on Android, or a JRiver MC client on another PC. It dishes out the MC library organized per my various MC views. If the server PC goes to sleep, then I can use an Android WOL app to wake it up over WiFi and proceed as described, without login. I think this is what the OP is asking for.
If in addition I install BubbleUPnP Server as a service and set “Startup Type: Automatic”, then I can use JRemote2, BubbleUPnP App, or an MC client to play to my Chromecast Audio dongles (they appear as DLNA renderers), all without login to the MC server PC.
This works without any additional 3rd party programs to set up JRiver Media Center 28 Service as a Windows service.
Caveat: My only experience is with audio media, essentially all ripped CDs or flac downloads, within a home network.