Note: The notepads are a separate collection so the related notes can only be accessed when that specific object is open. For example, to access the notepad on the constituent record, that record must be open. The sample applies to Action notepads.
If the Action record is open in the UI then the notepad can be accessed by using IBBnotepad, IBBAction2 and CAction objects. Here's sample code of how that would be accomplished using the ACTION_BEFORESAVE event in VBA.

'This procedure will be called before a record of the specified type is saved
Public Sub Action_BeforeSave(oRecord As Object, bCancel As Boolean)
    'oRecord              : record object being saved
    'bCancel              : set to true to cancel the save operation

    Dim oAction As CAction
    Dim oNotepad As IBBNotepad

    On Error GoTo ErrHandler

    Set oAction = oRecord 'Action record open
    
    Dim oAction2 As IBBAction2 'Collection of notepads associated with the action
    Set oAction2 = oAction

    If Not oAction Is Nothing Then
         'bCancel = < place your custom save criteria here >
         'MsgBox "Action_BeforeSave - Custom code is executed here"
        For Each oNotepad In oAction2.Notepads
          MsgBox "Actual Notes: " & oNotepad.Fields(NOTEPAD_fld_ActualNotes)
        Next oNotepad
        
    Set oAction2 = Nothing
    Set oNotepad = Nothing
    
    End If

    Set oAction = Nothing
    On Error GoTo 0
    Exit Sub

ErrHandler:
    Dim sErr As String
    sErr = Err.Description
    On Error GoTo 0
    '< place your custom error handling code here >
    MsgBox "Action_BeforeSave - Error processing Action_BeforeSave : " & sErr

    Set oAction = Nothing
    bCancel = False
    Exit Sub

End Sub

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