curl --request POST \
  --url https://api.hubapi.com/marketing/v3/marketing-events/events/{externalEventId}/cancel \
  --header 'Authorization: Bearer <token>'

{
  "startDateTime": "2023-11-07T05:31:56Z",
  "customProperties": [
    {
      "sourceId": "",
      "selectedByUser": false,
      "sourceLabel": "",
      "sourceUpstreamDeployable": "<string>",
      "source": "",
      "updatedByUserId": 123,
      "persistenceTimestamp": 123,
      "sourceMetadata": "",
      "dataSensitivity": "",
      "sourceVid": "",
      "unit": "",
      "requestId": "",
      "isEncrypted": false,
      "name": "",
      "useTimestampAsPersistenceTimestamp": true,
      "value": "",
      "selectedByUserTimestamp": 123,
      "timestamp": 123,
      "isLargeValue": true
    }
  ],
  "eventCancelled": true,
  "eventOrganizer": "<string>",
  "eventUrl": "<string>",
  "eventDescription": "<string>",
  "eventName": "<string>",
  "eventType": "<string>",
  "eventCompleted": true,
  "endDateTime": "2023-11-07T05:31:56Z",
  "objectId": "<string>"
}