public function subscribe($applicationName, $eventSource)
    {
        $uri = "applications/$applicationName/subscription";
        try {
            $response = $this->client->getEndpoint()->post($uri, [
                'form_params' => [
                    'eventSource' => $eventSource,
                ]
            ]);
        } catch (RequestException $e) {
            $this->processRequestException($e);
        }

        return new Application($this->client, \GuzzleHttp\json_decode($response->getBody()));
    }