You need to create a shared folder in the NAS if do not already have a suitable folder on the NAS.
The folder needs to be at least readable to ‘Every One’. Control Panel > Privilege > Shared Folder
In Volumio add the folder as a source: Settings > Sources > Add New Drive:
Alias - A short name for the qnap folder
NAS IP Address - e.g. 192.168.1.2
Path - Shared folder name
File Share Share Type - cifs
Apply/save