var unirest = require('unirest');
var accountSid = "XXXXXXXXX";
var accountToken = "YYYYYYYYY";
var authKey = "ZZZZZZZZZZ";
var encoding = Buffer.from(authKey + ':' + accountToken).toString('base64');
var req = unirest('POST', "https://<subdomain>/v2/accounts/"+accountSid+"/lists/+list_id+/contacts")
  .headers({
	  Authorization: 'Basic ' + encoding,
	  'Content-Type': 'application/json'
  })
  .send(JSON.stringify({"{   "contact_references": [     {"contact_sid":"d49b7a4781ea45dcb702825ce30494ba"},     {"contact_sid":"68bc98cae5a24cc3b017258a73e8a809"}   ] }"
))
  .end(function (res) { 
    if (res.error) throw new Error(res.error); 
    console.log(res.raw_body);
  });

<?php

$curl = curl_init();
$accountSid = "XXXXXXXXXX";
$accountToken = "YYYYYYYYYY";
$authKey = "ZZZZZZZZZZ";

$encoding = base64_encode($authKey .":". $accountToken);

curl_setopt_array($curl, array(
  CURLOPT_URL => 'https://<subdomain>/v2/accounts/<account_sid>/lists/<list_id>/contacts',
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => '',
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => 'POST',
  CURLOPT_POSTFIELDS =>'{   "contact_references": [     {"contact_sid":"d49b7a4781ea45dcb702825ce30494ba"},     {"contact_sid":"68bc98cae5a24cc3b017258a73e8a809"}   ] }',
  CURLOPT_HTTPHEADER => array(
    "Authorization: Basic ".$encoding,
    'Content-Type: application/json'
  ),
));

$response = curl_exec($curl);

curl_close($curl);
echo $response;

import requests
import base64 
import json
accountSid = "XXXXXXXXX"
authToken = "YYYYYYYYY"
authKey = "ZZZZZZZZZZ"

encoding = base64.b64encode(authKey + ":" + authToken)

url = "https://<subdomain>/v2/accounts/"+accountSid+"/lists/+list_id+/contacts"

payload = json.dumps({   "contact_references": [     {"contact_sid":"d49b7a4781ea45dcb702825ce30494ba"},     {"contact_sid":"68bc98cae5a24cc3b017258a73e8a809"}   ] })

headers = {
    'Authorization': "Basic " + encoding,
    'Content-Type': 'application/json'
}


response = requests.request("POST", url, data=payload, headers=headers)
print(response.text)
print(json.dumps(json.loads(response.text), indent = 4, sort_keys = True))

package main

import (
	b64 "encoding/base64"
"fmt"
"io/ioutil"
"net/http"
"strings"
)

func main() {
// Please provide accountSid, authToken from your Exotel account
	accountSid := "XXXXXXXXX"
	authToken := "YYYYYYYY"
	authKey := "ZZZZZZZZZZ"
// Encoding the accountSid and authToken, used in Authorization header
	encoding := b64.StdEncoding.EncodeToString([]byte(authKey + ":" + authToken))

	url := "https://<subdomain>/v2/accounts/" + accountSid + "/lists" + list_id + "/contacts"
	method := "POST"

	payload := strings.NewReader(`{   "contact_references": [     {"contact_sid":"d49b7a4781ea45dcb702825ce30494ba"},     {"contact_sid":"68bc98cae5a24cc3b017258a73e8a809"}   ] }`)

	client := &http.Client{}
	req, err := http.NewRequest(method, url, payload)
if err != nil {
		fmt.Println(err)
return
}
	req.Header.Add("Content-Type", "application/json")
	req.Header.Add("Authorization", "Basic "+encoding)
	res, err := client.Do(req)
if err != nil {
		fmt.Println(err)
return
}
	defer res.Body.Close()

	body, err := ioutil.ReadAll(res.Body)
if err != nil {
		fmt.Println(err)
return
}
	fmt.Println(string(body))
}