I was able to get the answer from the VFS helpline. There is another icon on the Index page prior to the application which allows upload of documents.

I have submitted an online application for OCI Registration in lieu of PIO card at: https://passport.gov.in/oci/capchaActionPIO I completed Parts A and B (including my photographs and signature). Unfortunately, before I was able to upload my supporting documentation i.e. passport / PIO / surrender c...