How to use external editor

Hi, I'd like to use an external editor with the arduino IDE, but when I click the "use external editor" in the settings window, all it does is disabling the coding area, I can't modify the content, nor paste code in it. Can anyone guide me on how to actually use an external editor with the IDE?

Open the same file as you loaded in the Arduino IDE with your external editor and edit away. After saving the file using your external editor and clicking verify/upload in the IDE the file will be loaded from disk again and compiled.

Thanks for your answer :)