curl --request GET \
  --url https://api.hubapi.com/feature-flags/v3/{appId}/flags/{flagName}/portals \
  --header 'Authorization: Bearer <token>'

{
  "portalFlagStates": [
    {
      "appId": 123,
      "flagName": "<string>",
      "flagState": "ABSENT",
      "portalId": 123
    }
  ]
}