An example based on the official docs:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: test-network-policy
  namespace: default
spec:
  podSelector:
    matchLabels:
      role: db
  policyTypes:
  - Egress
  egress:
  - to:
    - ipBlock:
        cidr: 10.11.12.13/32
    ports:
    - protocol: TCP
      port: 5978

It's essential to use /32 subnet prefix length which indicates that you're limiting the scope of the rule just to this one IP address.