Primeros pasos

using var http = new HttpClient();
http.DefaultRequestHeaders.Add("X-Api-Key", apiKey);
http.DefaultRequestHeaders.Add("Idempotency-Key", "factura-2026-0001");

string json = await File.ReadAllTextAsync("comprobante.json");
using var contenido = new StringContent(json, Encoding.UTF8, "application/json");
using var respuesta = await http.PostAsync(
    "https://staging-api.mynexusapi.com/v1/contribuyentes/0123456789/comprobantes", contenido);
Console.WriteLine($"{(int)respuesta.StatusCode} {await respuesta.Content.ReadAsStringAsync()}");

import json, os, requests
with open("comprobante.json", encoding="utf-8") as archivo:
    cuerpo = json.load(archivo)

r = requests.post(
    "https://staging-api.mynexusapi.com/v1/contribuyentes/0123456789/comprobantes",
    headers={"X-Api-Key": os.environ["ECUAFACT_API_KEY"], "Idempotency-Key": "factura-2026-0001"},
    json=cuerpo, timeout=30)
print(r.status_code, r.text)

import { readFile } from 'node:fs/promises';
const cuerpo = await readFile('comprobante.json', 'utf8');
const res = await fetch(
  'https://staging-api.mynexusapi.com/v1/contribuyentes/0123456789/comprobantes',
  {
    method: 'POST',
    headers: {
      'X-Api-Key': process.env.ECUAFACT_API_KEY,
      'Idempotency-Key': 'factura-2026-0001',
      'Content-Type': 'application/json'
    },
    body: cuerpo
  });
console.log(res.status, await res.text());

<?php
$ch = curl_init('https://staging-api.mynexusapi.com/v1/contribuyentes/0123456789/comprobantes');
curl_setopt_array($ch, [
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_POST => true,
    CURLOPT_HTTPHEADER => [
        'X-Api-Key: ' . getenv('ECUAFACT_API_KEY'),
        'Idempotency-Key: factura-2026-0001',
        'Content-Type: application/json',
    ],
    CURLOPT_POSTFIELDS => file_get_contents('comprobante.json'),
]);
echo curl_getinfo($ch, CURLINFO_HTTP_CODE) . ' ' . curl_exec($ch);

HttpClient cliente = HttpClient.newHttpClient();
String cuerpo = Files.readString(Path.of("comprobante.json"));
HttpRequest peticion = HttpRequest.newBuilder()
    .uri(URI.create("https://staging-api.mynexusapi.com/v1/contribuyentes/0123456789/comprobantes"))
    .header("X-Api-Key", System.getenv("ECUAFACT_API_KEY"))
    .header("Idempotency-Key", "factura-2026-0001")
    .header("Content-Type", "application/json")
    .POST(HttpRequest.BodyPublishers.ofString(cuerpo)).build();
HttpResponse<String> respuesta = cliente.send(peticion, HttpResponse.BodyHandlers.ofString());
System.out.println(respuesta.statusCode() + " " + respuesta.body());

string json = await http.GetStringAsync(
    "https://staging-api.mynexusapi.com/v1/operaciones/3f1c8c1e-6f2a-4b7e-9c3d-2a1b0c9d8e7f");
Console.WriteLine(json);

r = requests.get(
    "https://staging-api.mynexusapi.com/v1/operaciones/3f1c8c1e-6f2a-4b7e-9c3d-2a1b0c9d8e7f",
    headers={"X-Api-Key": os.environ["ECUAFACT_API_KEY"]}, timeout=30)
print(r.status_code, r.text)

const res = await fetch(
  'https://staging-api.mynexusapi.com/v1/operaciones/3f1c8c1e-6f2a-4b7e-9c3d-2a1b0c9d8e7f',
  { headers: { 'X-Api-Key': process.env.ECUAFACT_API_KEY } });
console.log(res.status, await res.text());

<?php
$ch = curl_init('https://staging-api.mynexusapi.com/v1/operaciones/3f1c8c1e-6f2a-4b7e-9c3d-2a1b0c9d8e7f');
curl_setopt_array($ch, [
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => ['X-Api-Key: ' . getenv('ECUAFACT_API_KEY')],
]);
echo curl_getinfo($ch, CURLINFO_HTTP_CODE) . ' ' . curl_exec($ch);

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;

HttpRequest peticion = HttpRequest.newBuilder()
    .uri(URI.create("https://staging-api.mynexusapi.com/v1/operaciones/3f1c8c1e-6f2a-4b7e-9c3d-2a1b0c9d8e7f"))
    .header("X-Api-Key", System.getenv("ECUAFACT_API_KEY"))
    .GET().build();
HttpClient cliente = HttpClient.newHttpClient();
System.out.println(cliente.send(peticion, HttpResponse.BodyHandlers.ofString()).body());