curl --request POST \
  --url 'https://api.flashcat.cloud/calendar/event/upsert?app_key=' \
  --header 'Content-Type: application/json' \
  --data '
{
  "cal_id": "<string>",
  "summary": "<string>",
  "is_off": true,
  "start_at": "<string>",
  "end_at": "<string>",
  "event_id": "<string>",
  "description": "<string>"
}
'

{
  "error": {
    "code": "<string>",
    "message": "<string>"
  },
  "data": {
    "cal_id": "<string>",
    "cal_name": "<string>"
  }
}