private function addAppSecretProofModifier($appSecret)
    {
        $accessToken = $this->getModifier(static::PARAM_ACCESS_TOKEN);
        if (!$accessToken) {
            return;
        }

        $this->modifiers([
          static::PARAM_APP_SECRET_PROOF => hash_hmac('sha256', $accessToken, $appSecret),
        ]);
    }