'Create and store a FakeWebHook.'

def add_webhook(self, **kwargs):

    allowed = ['scaling_group', 'policy', 'name', 'metadata']
    self._check_args(kwargs, allowed)
    webhook_id = str(next(self.webhook_counter))
    webhook = FakeWebHook(webhook_id, **kwargs)
    self.webhooks[webhook_id] = webhook
    return webhook
