code: public function addSecondaryDnsDomains($domain, $domain2add, $ip){
          $domain = (string)$domain;
          if (!$domain)
              throw new BadMethodCallException('Parameter $domain is missing.');
          $domain2add = (string)$domain2add;
          if (!$domain2add)
              throw new BadMethodCallException('Parameter $domain2add is missing.');
          $ip = (string)$ip;
          if (!$ip)
              throw new BadMethodCallException('Parameter $ip is missing.');
          $payload = array("domain"=>$domain2add, "ip"=>$ip);
          try {
              $r = $this->post('dedicated/server/'.$domain.'/secondaryDnsDomains', array('Content-Type' => 'application/json;charset=UTF-8'), json_encode($payload))->send();
          } catch (\Exception $e) {
              throw new ServerException($e->getMessage(), $e->getCode(), $e);
          }
      }
docs: Add a domain on secondary DNS
  
  @param string $domain
  @param string $domain2add
  @param string $ip
  @throws Exception\ServerException
  @throws \Ovh\Common\Exception\BadMethodCallException