Summarizing what i found out after a harrowing experience of 1 month back-and-forth with SAP Support channel who had no clue. Hope it helps.
For creating any ticket in Service Marketplace, a system is mandatory. Unfortunately the standard procedure of creating a system without license key doesnt work as there is no "HCP" in the Product dropdown field. So try this out:
Procedure:
1. As soon as you are onboarded, your system id should have been created with the HANA Cloud Installation Number (automatically).
2. Choose one of the components from http://service.sap.com/sap/support/notes/1888290
3. Send the incident
If you have an old account (dont ask me how old) or the above doesnt work as you cant find a system
1. Select any existing system (NW based for e.g.) from your list of systems in Service Marketplace
2. Choose one of the components from http://service.sap.com/sap/support/notes/1888290
3. Send the incident