HttpRequest request = HttpRequest.newBuilder()
        .uri(URI.create("https://api.messageflow.com/v2.1/sms"))
        .header("Content-Type", "application/json")
        .header("Accept", "application/json")
        .header("Authorization", "123")
        .header("Application-Key", "123")
        .method("POST", HttpRequest.BodyPublishers.ofString("{\n  \"sender\": \"string\",\n  \"message\": \"Twoja wiadomość testowa\",\n  \"phoneNumbers\": [\n    \"+48111222333\",\n    \"+48111222444\"\n  ],\n  \"phoneNumber\": \"+48111222333\",\n  \"validity\": 4320,\n  \"scheduleTime\": 0,\n  \"type\": 0,\n  \"shortLink\": true,\n  \"webhookUrl\": \"string\",\n  \"externalId\": \"xxxx-xxxx-xxxx\"\n}"))
        .build();

HttpResponse<String> response = HttpClient.newHttpClient().send(request, HttpResponse.BodyHandlers.ofString());
System.out.println(response.body());

import http.client

conn = http.client.HTTPSConnection("api.messageflow.com")

payload = "{\n  \"sender\": \"string\",\n  \"message\": \"Twoja wiadomość testowa\",\n  \"phoneNumbers\": [\n    \"+48111222333\",\n    \"+48111222444\"\n  ],\n  \"phoneNumber\": \"+48111222333\",\n  \"validity\": 4320,\n  \"scheduleTime\": 0,\n  \"type\": 0,\n  \"shortLink\": true,\n  \"webhookUrl\": \"string\",\n  \"externalId\": \"xxxx-xxxx-xxxx\"\n}"

headers = {
    'Content-Type': "application/json",
    'Accept': "application/json",
    'Authorization': "123",
    'Application-Key': "123"
}

conn.request("POST", "/v2.1/sms", payload, headers)

res = conn.getresponse()
data = res.read()

print(data.decode("utf-8"))

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.messageflow.com/v2.1/sms",
  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([
    'sender' => 'string',
    'message' => 'Twoja wiadomość testowa',
    'phoneNumbers' => [
        '+48111222333',
        '+48111222444'
    ],
    'phoneNumber' => '+48111222333',
    'validity' => 4320,
    'scheduleTime' => 0,
    'type' => 0,
    'shortLink' => null,
    'webhookUrl' => 'string',
    'externalId' => 'xxxx-xxxx-xxxx'
  ]),
  CURLOPT_HTTPHEADER => [
    "Accept: application/json",
    "Application-Key: 123",
    "Authorization: 123",
    "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"
"io"
"net/http"
"strings"
)

func main() {

url := "https://api.messageflow.com/v2.1/sms"

payload := strings.NewReader("{\n  \"sender\": \"string\",\n  \"message\": \"Hello world!\",\n  \"phoneNumbers\": [\n    \"+48111222333\",\n    \"+48111222444\"\n  ],\n  \"phoneNumber\": \"+48111222333\",\n  \"validity\": 4320,\n  \"scheduleTime\": 0,\n  \"type\": 0,\n  \"shortLink\": true,\n  \"webhookUrl\": \"string\",\n  \"externalId\": \"xxxx-xxxx-xxxx\"\n}")

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

req.Header.Add("Content-Type", "application/json")
req.Header.Add("Accept", "application/json")
req.Header.Add("Authorization", "123")
req.Header.Add("Application-Key", "123")

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

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

fmt.Println(res)
fmt.Println(string(body))

}

const response = await fetch('https://api.messageflow.com/v2.1/sms', {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Accept': 'application/json',
    'Authorization': 'YOUR_AUTHORIZATION_TOKEN',
    'Application-Key': 'YOUR_APPLICATION_KEY'
  },
  body: JSON.stringify({
    sender: 'YourCompany',
    message: 'Hello world!',
    phoneNumbers: ['+48111222333']
  })
});

const data = await response.json();

console.log(data);