$key='myApplicationKey';
$token = 'md5-string';
$domain='my-cool-shop.com';
$useHttps=true;

$transport  = new \InsalesApi\Transport(
    $key,
    $token,
    $domain,
    $useHttps
);
$api        = new \InsalesApi\InsalesAPI($transport);
$webhookApi = $api->webhook();

$webhook = $webhookApi->create('http://my-domain.com/callback.php',$webhookApi::MESSAGE_FORMAT_JSON);

concrete model properties are public

$id        = $webhook->id;
$format    = $webhook->format_type;
$createdAt = $webhook->created_at;
$topic     = $webhook->topic;
$address   = $webhook->address;

each response model has these methods

$lastHeaders          = $webhook->getHeaders();
$request              = $webhook->getRequest();
$originServerResponse = $webhook->getOriginData();