Use a button to perform a script that adds a new, blank record to the portal's table linked to the current parent record on your layout. This new record will then appear at the top of the portal (if you sort your portal accordingly) ready for adding a new portal record.
Set Variable [$ID ; LayoutTable::PrimaryKeyHere ]
Go to Layout ["PortalTable" (PortalTable) ]
Set Field [PortalTable::ForeignKeyHere ; $ID ]
Go to Layout [Original layout]
Thanks for the solution, got it partially working.
You are welcome to describe the part of this that is not working so that we can help you get it completely working.