Compose Key in Ubuntu

1 First install Tweaks by running

sudo apt install gnome-tweaks
# or
sudo apt install gnome-tweak-tool

2 Then launch Tweaks

And go to “Keyboard & Mouse” section. The “Compose Key” options is disabled by default. Click on the “Disabled” button.

3 Toggle the switch at the top on

And then you’ll be select the compose key. For example, Caps Lock.