You can't. That new blank record doesn't exist. What you are seeing is the standard "add" row for a portal with "allow creation of records via this relationship" enabled in the underlying relationship.
You can, however, put a button on your layout that, when clicked, performs a script that creates a new blank record. If you set up the sort order for your portal correctly, this new blank record will appear at the top of your record.
You can also place global fields corresponding to each field on the portal row just above the top of the portal, then a script, perhaps performed by a script trigger, can take the data in the global fields and add it to your portal's table. This would look and function much like having a an "add" row at the top of your portal.