$ composer require guzzlehttp/guzzle

<?php
require_once('vendor/autoload.php');

$client = new \GuzzleHttp\Client();

$response = $client->request('GET', 'https://api-test.ticketbaiws.eus/tbai-pdf/', [
  'body' => '{"serie":"A","numero":"2021000123"}',
  'headers' => [
    'Accept' => 'application/json',
    'Content-Type' => 'application/json',
    'Token' => 'xxx',
    'Nif' => '00000014Z'
  ],
]);

echo $response->getBody();

$ python -m pip install requests

import requests

url = "https://api-test.ticketbaiws.eus/tbai-pdf/"
headers = {"Accept": "application/json", "Token" : "xxx", "Nif" : "00000014Z"}
response = requests.request("GET", url, headers=headers)
print(response.text)

CURL *hnd = curl_easy_init();

curl_easy_setopt(hnd, CURLOPT_CUSTOMREQUEST, "GET");
curl_easy_setopt(hnd, CURLOPT_URL, "https://api-test.ticketbaiws.eus/tbai-pdf/");

struct curl_slist *headers = NULL;
headers = curl_slist_append(headers, "Accept: application/json");
curl_easy_setopt(hnd, CURLOPT_HTTPHEADER, headers);

CURLcode ret = curl_easy_perform(hnd);

OkHttpClient client = new OkHttpClient();

Request request = new Request.Builder()
  .url("https://api-test.ticketbaiws.eus/tbai-pdf/?serie=A&numero=2021000123")
  .get()
  .addHeader("Accept", "application/json")
  .addHeader("Token", "xxx")
  .addHeader("Nif", "00000014Z")
  .build();
  
Response response = client.newCall(request).execute();