curl --request POST \
  --url https://api.hubapi.com/conversations/v3/custom-channels/{channelId}/messages \
  --header 'Content-Type: application/json' \
  --header 'private-app: <api-key>' \
  --data '{
  "messageDirection": "INCOMING",
  "attachments": [
    {
      "fileUsageType": "<string>",
      "type": "FILE",
      "fileId": "<string>"
    }
  ],
  "preResolvedContacts": {
    "contacts": [
      {
        "contactVid": 123,
        "contactPropertiesLeadingToMatch": [
          "<string>"
        ]
      }
    ]
  },
  "recipients": [
    {
      "name": "<string>",
      "deliveryIdentifier": {
        "type": "<string>",
        "value": "<string>"
      }
    }
  ],
  "integrationThreadId": "<string>",
  "integrationIdempotencyId": "<string>",
  "text": "<string>",
  "richText": "<string>",
  "channelAccountId": "<string>",
  "senders": [
    {
      "name": "<string>",
      "deliveryIdentifier": {
        "type": "<string>",
        "value": "<string>"
      }
    }
  ],
  "inReplyToId": "<string>",
  "timestamp": "2023-11-07T05:31:56Z"
}'

{
  "attachments": [
    {
      "fileUsageType": "<string>",
      "name": "<string>",
      "type": "FILE",
      "url": "<string>",
      "fileId": "<string>"
    }
  ],
  "subject": "<string>",
  "conversationsThreadId": "<string>",
  "type": "MESSAGE",
  "richText": "<string>",
  "inReplyToId": "<string>",
  "createdAt": "2023-11-07T05:31:56Z",
  "archived": true,
  "createdBy": "<string>",
  "recipients": [
    {
      "actorId": "<string>",
      "name": "<string>",
      "deliveryIdentifier": {
        "type": "<string>",
        "value": "<string>"
      },
      "recipientField": "<string>"
    }
  ],
  "truncationStatus": "NOT_TRUNCATED",
  "client": {
    "clientType": "HUBSPOT",
    "integrationAppId": 123
  },
  "id": "<string>",
  "text": "<string>",
  "channelAccountId": "<string>",
  "senders": [
    {
      "actorId": "<string>",
      "name": "<string>",
      "senderField": "<string>",
      "deliveryIdentifier": {
        "type": "<string>",
        "value": "<string>"
      }
    }
  ],
  "channelId": "<string>",
  "updatedAt": "2023-11-07T05:31:56Z",
  "status": {
    "statusType": "SENT",
    "failureDetails": {
      "errorMessageTokens": {},
      "errorMessage": "<string>"
    }
  },
  "direction": "INCOMING"
}