Commissionerate code for gst registration

How to select the commissionerate Code while registering for GST on gst.gov.in

We have Service Tax Registration at Service Tax - I, DIvision - III, Mumbai however this is not available on GST.

I have sent an email to helpdesk @ gst.gov.in but there is no revert from them event after a week.

Anybody here knows how to resolve this?

Thanks in advance :)