I just give a try to run the service in GUI, then had a look at the apisvc logs:  Request: POST - [URL] - namespace=UNMANAGED_VOLUMES from 127.0.0.1  So it seems you're almost on track. You just need to enter the URN of the StorageSystem you want to discover the Unmanaged Volumes.
  I am trying to figure out whats the REST API call for performing the catalog action "Discover Unmanaged Volumes".  I found this in the REST API guide : POST /vdc/storage-systems/discover but it doesn't seem to be updating unmanaged volumes information on VIPR controller. It just performs a quick discover that lasts about 2 minutes per VMAX.  I've tried to test above API call and tried to ingest volumes. But the unmanaged volumes are not getting updated. However when i use catalog item "Discover Unmanaged Volumes", i was able to perform ingest successfully.  I'm on VIPR controller version 3.5  Following document doesn't have API method to perform this task. [URL]   Appreciate your help. 
What the REST API call for discover unmananged volumes ? VIPR 3.5