File this under: Useful for me. It’s another of those things that is easy when you know how but not entirely intuitive.
From the System Settings app, select Keyboard (or just launch the Keyboard app directly from the shell) and click on Layout Settings.
And select Options…
Expand the Adding currency signs to certain keys and select Euro on 5
Now for the less than intuitive bit. Expand the Key to choose 3rd level and select Right Alt
Close and exit. And now you can type the € by pressing
Alt Gr + 5.