In order to create a new row in a portal, you must select "Allow creation of records in this table via this Relationship" when you set up the relationship.
Then your portal will 'ghost' a new record on the next row, depending on your field settings, enabling you to enter data to create a new record in the related table
You don't have to enable this option. Some developers prefer not to. The alternative is to use a button with a script to create a new record in the portal's table.