NSComboBox: how to tell the user to type information that is not in the pop-up list and read it

I have an NSComboBox with a separate class that matches the NSComboBox dataSource and delegate.

If the user enters text in a combo box that does not correspond to one of the items in the pop-up list, how to get the notification that the user typed, as well as read the value entered by the user

I am currently implementing delegate methods: -

-(void)comboBoxSelectionDidChange:(NSNotification *)notification 
-(void)comboBoxWillDismiss:(NSNotification *)notification 

However, they are displayed only for notifications when the user selects an item from a pop-up window, and not when the user types something else when stopping editing