Windows: press `Alt` + `0128` on the numeric keypad
Windows: press `Ctrl` + `Alt` + `E`
Mac: press `Option` + `Shift` + `2`
Linux: press `Ctrl` + `Shift` + `U`, type `20AC`, then press `Enter`
iPhone/iPad: press and hold `$`, then select `€`
Android: press and hold `$`, then select `€`
UK keyboard: press `AltGr` + `4`
Some keyboards: use the `€` key if available