PARENT_NAMESPACE=pizzeria
CHILD_NAMESPACE=pepperoni

$ kubectl annotate namespace $CHILD_NAMESPACE \ 
  hnc.x-k8s.io/subnamespace-of=$PARENT_NAMESPACE
  
# namespace/pepperoni annotated

$ cat <<EOF | kubectl apply -f -
apiVersion: hnc.x-k8s.io/v1alpha2
kind: SubnamespaceAnchor
metadata:
  namespace: $PARENT_NAMESPACE
  name: $CHILD_NAMESPACE
EOF

# subnamespaceanchor.hnc.x-k8s.io/pepperoni created