curl --request POST \
  --url https://api.hubapi.com/email/public/v1/singleEmail/send \
  --header 'Content-Type: application/json' \
  --data '
{
  "emailId": 123,
  "message": {
    "to": "<string>",
    "from": "<string>",
    "sendId": "<string>",
    "replyTo": "<string>",
    "replyToList": [
      "<string>"
    ],
    "cc": [
      "<string>"
    ],
    "bcc": [
      "<string>"
    ]
  },
  "contactProperties": [
    {
      "name": "<string>",
      "value": "<string>"
    }
  ],
  "customProperties": [
    {
      "name": "<string>",
      "value": "<string>"
    }
  ]
}
'

{
  "sendResult": "SENT",
  "message": "<string>",
  "eventId": {
    "id": "<string>",
    "created": 123
  }
}