It is in the link I posted above for you. The extensions example.
You will not be able to do this in the GUI, you will need to use the extensions_override.conf file in /etc/asterisk if you would like to use the extension you have programmed in the GUI. The override file will override the behavior of the extension as it is programmed in the GUI. This is useful if you want to see the extension exists in the GUI, but need it to do something more custom.