Hosting Servers Using Private Workshop Files
Unfortunately, the dedicated server cannot login to a Steam account to download private workshop files. So, a workaround is necessary.
By logging into steamcmd you can download the workshop file using your account:
login (credentials)
workshop_download_item 304930 (workshopfileid)
This will download the file into the steamcmd folder at steamapps/workshop/content/304930/(workshopfileid).
You can now create a symbolic link between the server and the workshop file.
For example on Linux testing map MyMap, open the U3DS folder and use:
ln -S ../../workshop/content/304930/(workshopfileid)/MyMap Maps/MyMap
The workshop file ID should still be added to WorkshopDownloadConfig.json so that clients enable it when joining.