code: function addCustomProperties($mailingId, $properties) {
  
          $xml = new SimpleXMLElement("<?xml version=\"1.0\"?><properties></properties>");
  
          if (is_array($properties)) {
              foreach ($properties as $property) {
                  $this->sxml_append($xml, $property->toXML());
              }
          } else {
              $this->sxml_append($xml, $properties->toXML());
          }
  
          return $this->post("mailings/${mailingId}/settings/properties", $xml->asXML());
      }
docs: Adds a list of custom properties to the mailing with the provided id.
  
  @param int $mailingId
  @param array $properties Array of com_maileon_api_mailings_CustomProperty or single property
  @return com_maileon_api_MaileonAPIResult