Construa sobre a API do WhatsApp.

await fetch('https://api.wassenger.com/v1/messages', {
  method: 'POST',
  headers: {
    'Token': process.env.WASSENGER_API_KEY,
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({
    phone: '+1234567890',
    message: 'Hello from Wassenger'
  })
})

import os, requests

requests.post(
    'https://api.wassenger.com/v1/messages',
    headers={'Token': os.environ['WASSENGER_API_KEY']},
    json={
        'phone': '+1234567890',
        'message': 'Hello from Wassenger'
    }
)

$ch = curl_init('https://api.wassenger.com/v1/messages');
curl_setopt_array($ch, [
    CURLOPT_POST       => true,
    CURLOPT_HTTPHEADER => [
        'Token: ' . getenv('WASSENGER_API_KEY'),
        'Content-Type: application/json'
    ],
    CURLOPT_POSTFIELDS => json_encode([
        'phone'   => '+1234567890',
        'message' => 'Hello from Wassenger'
    ])
]);
curl_exec($ch);