public function pushMessages($targetType, array $messages, $requestId = null, $campaignKey = null)
    {
        $data = [
            'request_id' => $requestId,
            'campaign_key' => $campaignKey,
            'target_type' => $targetType,
            'messages' => $messages,
        ];

        $options = [
            'auth' => [
                $this->apiKey,
                $this->apiSecret,
            ],
            'json' => $data,
        ];
        $uri = rtrim($this->uri, '/') . '/' . $this->appID;
        $response = $this->client->post($uri, $options);
        return json_decode($response->getBody(), true);
    }