API de WhatsApp con C# y .NET

using System.Net.Http.Json;

using var http = new HttpClient { Timeout = TimeSpan.FromSeconds(30) };

var respuesta = await http.PostAsJsonAsync("https://api.kontacto.pro/send-message", new
{
    api_key = Environment.GetEnvironmentVariable("WSP_API_KEY"),
    sender = "51999888777",
    number = "51987654321",
    message = "Hola desde C#"
});

var datos = await respuesta.Content.ReadFromJsonAsync<Respuesta>();
Console.WriteLine(datos?.status == true ? "Enviado" : $"No se envió: {datos?.msg}");

record Respuesta(bool status, string msg);

await http.PostAsJsonAsync("https://api.kontacto.pro/send-media", new
{
    api_key = Environment.GetEnvironmentVariable("WSP_API_KEY"),
    sender = "51999888777",
    number = "51987654321",
    media_type = "pdf",
    caption = "F001-2045.pdf",
    url = "https://tusistema.com/comprobantes/F001-2045.pdf"
});