chromium/chromeos/components/test/data/onc/policy/policy_allow_apn_modification.onc

{
  "NetworkConfigurations": [
    {
      "GUID": "policy_cellular",
      "Type": "Cellular",
      "Name": "Managed cellular",
      "Cellular": {
        "SMDPAddress": "1$SMDP.GSMA.COM$00000-00000-00000-000-0001"
      }
    }
  ],
  "GlobalNetworkConfiguration": {
    "AllowAPNModification": false
  },
  "Type": "UnencryptedConfiguration"
}