curl --request POST \
  --url https://rest-api.smsmngr.com/v1/billing/invoices/pdf \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '
{
  "billing_id": "1234",
  "invoice_id": "123",
  "lang": "cs"
}
'

import requests

url = "https://rest-api.smsmngr.com/v1/billing/invoices/pdf"

payload = {
    "billing_id": "1234",
    "invoice_id": "123",
    "lang": "cs"
}
headers = {
    "x-api-key": "<api-key>",
    "Content-Type": "application/json"
}

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

print(response.text)

const options = {
  method: 'POST',
  headers: {'x-api-key': '<api-key>', 'Content-Type': 'application/json'},
  body: JSON.stringify({billing_id: '1234', invoice_id: '123', lang: 'cs'})
};

fetch('https://rest-api.smsmngr.com/v1/billing/invoices/pdf', 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://rest-api.smsmngr.com/v1/billing/invoices/pdf",
  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([
    'billing_id' => '1234',
    'invoice_id' => '123',
    'lang' => 'cs'
  ]),
  CURLOPT_HTTPHEADER => [
    "Content-Type: application/json",
    "x-api-key: <api-key>"
  ],
]);

$response = curl_exec($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://rest-api.smsmngr.com/v1/billing/invoices/pdf"

	payload := strings.NewReader("{\n  \"billing_id\": \"1234\",\n  \"invoice_id\": \"123\",\n  \"lang\": \"cs\"\n}")

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

	req.Header.Add("x-api-key", "<api-key>")
	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://rest-api.smsmngr.com/v1/billing/invoices/pdf")
  .header("x-api-key", "<api-key>")
  .header("Content-Type", "application/json")
  .body("{\n  \"billing_id\": \"1234\",\n  \"invoice_id\": \"123\",\n  \"lang\": \"cs\"\n}")
  .asString();

require 'uri'
require 'net/http'

url = URI("https://rest-api.smsmngr.com/v1/billing/invoices/pdf")

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

request = Net::HTTP::Post.new(url)
request["x-api-key"] = '<api-key>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"billing_id\": \"1234\",\n  \"invoice_id\": \"123\",\n  \"lang\": \"cs\"\n}"

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

{
  "success": true,
  "invoice": "JVBERi0xLjQKJ...=="
}

{
  "message": "Missing 'billing_id' or 'invoice_id' in request data"
}

{
  "message": "Apikey is not authorized for this billing_id"
}

{
  "message": "Failed to get invoice"
}