You could go to your relationship dialog "File" - "Manage" - "Database" and click the third tab "Relationships".
Then doube click the relationship between your Main and your notes. In the new dialogue box select "Allow creation of records via this relationshop" on the side of the notes table.
This way you can add notes directly from within the portal.
The other way would be to make a script where you:
- Set variable [$MainID; Main::ID]
- Go to Layout (Notes)
- Create New Record
- Set Field [Notes::MaiIdFk; $MainId]
Then you manually type your note after wich you might need a button to go back to the main layout.
But I would go woth the first option.