Script triggers don't work in IWP and you don't need a trigger for this in regular FileMaker. You can edit any existing record in the portal simply by entering the fields and editing them like any other fields. If "Allow creation of records via this relationship..." is enabled for the portal's table occurrence, you can even add new records to the portal.
I don't recall off hand, however, whether you can add new records directly in the portal this way in IWP. You may need to add a "new row" button that creates the new record in the portal's table and then refreshes to reveal it in the portal.
Thanks Phil. I've added a button for users to activate the portal and that works in IWP mode. A small script was created to achieve this in IWP mode:
- Enter Browse Mode 
- Go to Portal Record [Select; First]