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

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