https://kubernetes.io/docs/concepts/services-networking/network-policies/#targeting-multiple-namespaces-by-label

In this scenario, your Egress NetworkPolicy targets more than one namespace using their label names. For this to work, you need to label the target namespaces. For example:

kubectl label namespace frontend namespace=frontend
kubectl label namespace backend namespace=backend

Add the labels under namespaceSelector in your NetworkPolicy document. For example:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: egress-namespaces
spec:
  podSelector:
    matchLabels:
      app: myapp
  policyTypes:
  - Egress
  egress:
  - to:
    - namespaceSelector:
        matchExpressions:
        - key: namespace
          operator: In
          values: ["frontend", "backend"]

https://kubernetes.io/docs/concepts/services-networking/network-policies/#targeting-a-namespace-by-its-name

The Kubernetes control plane sets an immutable label kubernetes.io/metadata.name on all namespaces, the value of the label is the namespace name.

While NetworkPolicy cannot target a namespace by its name with some object field, you can use the standardized label to target a specific namespace.