This is a fileamker 11 only solution. There are different approaches that work with older versions.
You need a serial number or other field in the portal table that uniquely identifies each record.
Define a filtered portal to your records with this filter expression:
$$RecordSerial = PortalTable::SerialID
Write a script:
Set Variable [ $$RecordSerial ; Value: PortalTable::SerialID ]
Refresh Window [ Flusch cached Join results]
Put a button in your first portal's row or use button setup to define a field or group of fields in the portal row to perform this script.