WhatsApp API for billing and ERP software: send the invoice in one call

C# (.NET 6 or later)

using System.Text.Json;

var http = new HttpClient { BaseAddress = new Uri("https://talkback.js1software.net/") };
http.DefaultRequestHeaders.Authorization = new("Bearer", apiKey);

using var form = new MultipartFormDataContent();
form.Add(new StringContent(JsonSerializer.Serialize(new
{
    to = "919876543210", template = "invoice_pdf",
    @params = new[] { "Ramesh Textiles", "INV-1043" }, reference = "INV-1043"
})), "message");
var pdf = new ByteArrayContent(await File.ReadAllBytesAsync("INV-1043.pdf"));
pdf.Headers.ContentType = new("application/pdf");
form.Add(pdf, "document", "INV-1043.pdf");

var req = new HttpRequestMessage(HttpMethod.Post, "v1/messages") { Content = form };
req.Headers.Add("Idempotency-Key", "INV-1043-send-1"); // reuse it on every retry
var res = await http.SendAsync(req);                   // 202 = accepted: save the id

PHP (curl)

$ch = curl_init('https://talkback.js1software.net/v1/messages');
curl_setopt_array($ch, [
    CURLOPT_POST           => true,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER     => ['Authorization: Bearer ' . $apiKey,
                               'Idempotency-Key: INV-1043-send-1'],
    CURLOPT_POSTFIELDS     => [
        'message'  => json_encode(['to' => '919876543210',
                        'template' => 'invoice_pdf',
                        'params' => ['Ramesh Textiles', 'INV-1043'],
                        'reference' => 'INV-1043']),
        'document' => new CURLFile('INV-1043.pdf', 'application/pdf', 'INV-1043.pdf'),
    ],
]);
$body   = curl_exec($ch);
$status = curl_getinfo($ch, CURLINFO_RESPONSE_CODE); // 202 = accepted