public function add(string $requestMessage = null) : void
    {
        if ($this->onlineIdParameter() === 'me') return;

        $data = ($requestMessage === null) ? new \stdClass() : [
            "requestMessage" => $requestMessage
        ];

        $this->postJson(sprintf(self::USERS_ENDPOINT . 'friendList/%s', $this->client->onlineId(), $this->onlineId()), $data);
    }