$ composer require guzzlehttp/guzzle

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

$client = new \GuzzleHttp\Client();

$response = $client->request('POST', 'https://api-test.ticketbaiws.eus/tbai-completar/', [
  'body' => '{"fecha":"13\/11\/2021","hora":"12:14:00","nif":"B00000011","nombre":"Empresa de ejemplo S.L.","direccion":"Calle Falsa 123","cp":"28080","serie":"A-","numero":"2021000123","intracomunitaria":false,"simplificadas":[{"serie":"A","numero": "20210000075","fecha": "10\/11\/2021"}]}',
  '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-completar/"
payload = {
    "fecha": "13\/11\/2021",
    "hora": "12:14:00",
    "pais_cliente": "ES",
	"nif": "B00000011",
    "nombre": "Empresa de ejemplo S.L.",
    "direccion": "Calle Falsa 123",
    "cp": "28080",
    "serie": "A",
    "numero": "2021000123",
    "simplificadas" : [
        {
            "serie": "A",
            "numero": "20210000075",
            "fecha": "10\/11\/2021"
        }
    ],
    "intracomunitaria": false,
	"exportacion": false
}

headers = {"Accept": "application/json", "Token" : "xxx", "Nif" : "00000014Z"}
response = requests.request("POST", url, json=payload, headers=headers)

print(response.text)

CURL *hnd = curl_easy_init();

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

struct curl_slist *headers = NULL;
headers = curl_slist_append(headers, "Accept: application/json");
curl_easy_setopt(hnd, CURLOPT_HTTPHEADER, headers);
curl_easy_setopt(hnd, CURLOPT_POSTFIELDS, '{"fecha":"13\/11\/2021","hora":"12:14:00","nif":"B00000011","nombre":"Empresa de ejemplo S.L.","direccion":"Calle Falsa 123","cp":"28080","serie":"A-","numero":"2021000123","intracomunitaria":false,"simplificadas":[{"serie":"A","numero": "20210000075","fecha": "10\/11\/2021"}]}');

CURLcode ret = curl_easy_perform(hnd);

OkHttpClient client = new OkHttpClient();

MediaType mediaType = MediaType.parse("application/json");
RequestBody body = RequestBody.create(mediaType, '{"fecha":"13\/11\/2021","hora":"12:14:00","nif":"B00000011","nombre":"Empresa de ejemplo S.L.","direccion":"Calle Falsa 123","cp":"28080","serie":"A-","numero":"2021000123","intracomunitaria":false,"simplificadas":[{"serie":"A","numero": "20210000075","fecha": "10\/11\/2021"}]}');

Request request = new Request.Builder()
  .url("https://api-test.ticketbaiws.eus/tbai-completar/")
  .post(body)
  .addHeader("Accept", "application/json")
  .addHeader("Token", "xxx")
  .addHeader("Nif", "00000014Z")
  .build();
  
Response response = client.newCall(request).execute();