code: private function buildRequestHeaders($body)
      {
          $credentials = $this->getConfig('credentials');
          $appId = $credentials->getAppId();
          $certId = $credentials->getCertId();
  
          $headers = [];
  
          $headers['Accept'] = 'application/json';
          $headers['Authorization'] = 'Basic '.base64_encode($appId.':'.$certId);
          $headers['Content-Type'] = 'application/x-www-form-urlencoded';
          $headers['Content-Length'] = strlen($body);
  
          return $headers;
      }
docs: Helper function that builds the HTTP request headers.
  
  @param string $body The request body.
  
  @return array An associative array of HTTP headers.