Pues esto me "funciona":
Código PHP:
Ver original<?php
require_once 'vendor/autoload.php';
$client = new GuzzleHttp\Client();
try {
//if using Guzzle 6+ change "body" to "form_params"
$response = $client->post('https://www.twitchalerts.com/api/v1.0/donations', [
'form_params' => [
'access_token' => 'ACCESS_TOKEN_HERE',
'name' => 'Thomas',
'amount' => 50,
'currency' => 'USD',
'message' => 'nice!'
]
]);
$result = $response->json();
} catch (Exception $e) {
var_dump($e->getResponse()->getStatusCode());
//failed
// $result = $e->getResponse()->json();
}