Port Code for Services

I have received an email every month with subject "non-transmission of invoices due to negative balance".
then I have checked on Portal and there is showing the reason as your Port code mismatched.

I have booked export invoice Table 3.1(b) for services without port code as there is no port code for services.

I want to know that what is the port code for Services under GST.

please guide me and help me out.

Thanks in advance