curl --request POST \
  --url https://http.us.breakcold.com/api/v1/records/{recordId}/custom-activities \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "title": "<string>",
  "descriptionHtml": "<string>",
  "descriptionPlain": "<string>",
  "startAt": 123,
  "endAt": 123,
  "linkedRecordIds": [
    "<string>"
  ],
  "participantUserIds": [
    "<string>"
  ]
}
'

{
  "activity": {
    "id": "<string>",
    "workspaceId": "<string>",
    "title": "<string>",
    "startAt": 123,
    "linkedRecordIds": [
      "<string>"
    ],
    "participantUserIds": [
      "<string>"
    ],
    "createdAt": 123,
    "updatedAt": 123,
    "descriptionHtml": "<string>",
    "descriptionPlain": "<string>",
    "endAt": 123,
    "primaryRecordId": "<string>",
    "createdByUserId": "<string>"
  }
}