Your Namespace workspace (project/tenant) id — the segment between the registry host and the repository name:

nscr.io/abc012abc012/jenkins-agent:1.0

Find it with nsc registry list, or read it off any image reference you have already pushed.

Why this cannot be detected automatically: the registry API returns bare repository names and the tenant is implicit in the token. Every RPC that would reveal the tenant id requires an admin or partner token, which a least-privileged Jenkins token deliberately is not.

Leave it blank and the plugin still works, but the Agent image suggestion list produces references without the workspace segment, which Namespace rejects with failed to resolve image.