curl --request POST \
  --url https://api.hubapi.com/feature-flags/v3/{appId}/flags/{flagName}/portals/batch/upsert \
  --header 'Content-Type: application/json' \
  --data '{
  "portalStates": [
    {
      "portalId": 123,
      "flagState": "OFF"
    }
  ]
}'

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