Service Tax Registration amendment

We want to add one more service to our registration certificate. We have service tax login id and password. Service tax office refused to take request letter with original certiifcate for inclusion of additional service. They advised to amend application on line. I could not find any provision in the online to amend the ST1 application form or ST2 certificate to include additional service in the registration.

I will be highly thankful if anybody can guide me how to include additional service in the certificate.

Thanks in advance