$ composer require guzzlehttp/guzzle

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

$client = new \GuzzleHttp\Client();

$response = $client->request('POST', 'https://api.ticketbaiws.eus/doc-representante/', [
    'headers' => [
        'Accept' => 'application/json',
        'Content-Type' => 'application/json',
        'Token' => 'xxx',
        'Nif' => '00000014Z'
    ],
    'multipart' => [
        [
            'name'     => 'file',
            'contents' => fopen('/ruta/documento-firmado.pdf', 'r'),
            'filename' => 'documento-firmado.pdf'
        ]
    ]
]);

echo $response->getBody();

$ python -m pip install requests

import requests

url = "https://api.ticketbaiws.eus/doc-representante/"

headers = {"Accept": "application/json", "Token" : "xxx", "Nif" : "00000014Z"}

files = {
    "file": ("document-firmado.pdf", open("/ruta/documento-firmado.pdf", "rb"), "application/pdf")
}

response = requests.post(url, headers=headers, files=files)

print(response.text)

CURL *hnd = curl_easy_init();

curl_easy_setopt(hnd, CURLOPT_CUSTOMREQUEST, "POST");
curl_easy_setopt(hnd, CURLOPT_URL, "https://api.ticketbaiws.eus/doc-representante/");

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

// -------------------------
//    FORMULARIO MULTIPART
// -------------------------

curl_mime *mime;
curl_mimepart *part;

mime = curl_mime_init(hnd);

// Campo "filename"
part = curl_mime_addpart(mime);
curl_mime_name(part, "filename");
curl_mime_data(part, "documento-firmado.pdf", CURL_ZERO_TERMINATED);

// Campo "file" → fichero real
part = curl_mime_addpart(mime);
curl_mime_name(part, "file");
curl_mime_filedata(part, "/ruta/documento-firmado.pdf");

// Asignar formulario al handle
curl_easy_setopt(hnd, CURLOPT_MIMEPOST, mime);

CURLcode ret = curl_easy_perform(hnd);

OkHttpClient client = new OkHttpClient();

MediaType mediaType = MediaType.parse("application/json");

// Archivo a enviar
File file = new File("/ruta/documento-firmado.pdf");

// Construimos el body multipart
RequestBody requestBody = new MultipartBody.Builder()
    .setType(MultipartBody.FORM)
    // Campo "file"
    .addFormDataPart(
            "file",                   // nombre del campo
            file.getName(),           // nombre del archivo
            RequestBody.create(file, MediaType.parse("application/pdf"))
    )
    // Puedes agregar más campos si el endpoint lo requiere
    .addFormDataPart("filename", file.getName())
    .build();

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