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>"
    }
  ]
}
'

import requests

url = "https://api.hubapi.com/email/public/v1/singleEmail/send"

payload = {
    "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>"
        }
    ]
}
headers = {"Content-Type": "application/json"}

response = requests.post(url, json=payload, headers=headers)

print(response.text)

const options = {
  method: 'POST',
  headers: {'Content-Type': 'application/json'},
  body: JSON.stringify({
    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>'}]
  })
};

fetch('https://api.hubapi.com/email/public/v1/singleEmail/send', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.hubapi.com/email/public/v1/singleEmail/send",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_POSTFIELDS => json_encode([
    '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>'
        ]
    ]
  ]),
  CURLOPT_HTTPHEADER => [
    "Content-Type: application/json"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://api.hubapi.com/email/public/v1/singleEmail/send"

	payload := strings.NewReader("{\n  \"emailId\": 123,\n  \"message\": {\n    \"to\": \"<string>\",\n    \"from\": \"<string>\",\n    \"sendId\": \"<string>\",\n    \"replyTo\": \"<string>\",\n    \"replyToList\": [\n      \"<string>\"\n    ],\n    \"cc\": [\n      \"<string>\"\n    ],\n    \"bcc\": [\n      \"<string>\"\n    ]\n  },\n  \"contactProperties\": [\n    {\n      \"name\": \"<string>\",\n      \"value\": \"<string>\"\n    }\n  ],\n  \"customProperties\": [\n    {\n      \"name\": \"<string>\",\n      \"value\": \"<string>\"\n    }\n  ]\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

HttpResponse<String> response = Unirest.post("https://api.hubapi.com/email/public/v1/singleEmail/send")
  .header("Content-Type", "application/json")
  .body("{\n  \"emailId\": 123,\n  \"message\": {\n    \"to\": \"<string>\",\n    \"from\": \"<string>\",\n    \"sendId\": \"<string>\",\n    \"replyTo\": \"<string>\",\n    \"replyToList\": [\n      \"<string>\"\n    ],\n    \"cc\": [\n      \"<string>\"\n    ],\n    \"bcc\": [\n      \"<string>\"\n    ]\n  },\n  \"contactProperties\": [\n    {\n      \"name\": \"<string>\",\n      \"value\": \"<string>\"\n    }\n  ],\n  \"customProperties\": [\n    {\n      \"name\": \"<string>\",\n      \"value\": \"<string>\"\n    }\n  ]\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://api.hubapi.com/email/public/v1/singleEmail/send")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Content-Type"] = 'application/json'
request.body = "{\n  \"emailId\": 123,\n  \"message\": {\n    \"to\": \"<string>\",\n    \"from\": \"<string>\",\n    \"sendId\": \"<string>\",\n    \"replyTo\": \"<string>\",\n    \"replyToList\": [\n      \"<string>\"\n    ],\n    \"cc\": [\n      \"<string>\"\n    ],\n    \"bcc\": [\n      \"<string>\"\n    ]\n  },\n  \"contactProperties\": [\n    {\n      \"name\": \"<string>\",\n      \"value\": \"<string>\"\n    }\n  ],\n  \"customProperties\": [\n    {\n      \"name\": \"<string>\",\n      \"value\": \"<string>\"\n    }\n  ]\n}"

response = http.request(request)
puts response.read_body

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

{
  "category": "<string>",
  "correlationId": "<string>",
  "message": "<string>",
  "subCategory": "<string>",
  "errors": [
    {
      "message": "<string>",
      "in": "<string>",
      "code": "<string>",
      "subCategory": "<string>",
      "context": {}
    }
  ],
  "context": {},
  "links": {}
}