Each portal row is a record in a related table. If you open Manage | Database | Relationships, you can find the Tutorial: What are Table Occurrences? for your layout and the table occurrence for you portal. If you double click the line that connects them, you can select "allow creation of records via this relationship" for the portal's table occurrence. This will add an extra blank row below the last record in your portal. If you enter data into a field in this portal row FileMaker will automatically create and link a new record in the portal's table.
Not all relationships that might be used for a portal permit this option and some developers prefer to use a button for adding new portal records to keep from having to scroll the portal to expose that blank "add" row each time they need to add a portal record. A button for this uses a script to create a new record in the portal's table and link it to the current record on your layout.
Thank you Phil! Works perfectly now!