Since I always forget how to add the EurKEY layout after installing a new system every now and then, I once for all gonna write it down here for my future me:

• Install the gnome-tweaks package: sudo apt install gnome-tweaks
• Open Tweaks, select Keyboard & Mouse, and enable Show Extended Input Sources.
• Important: Log out and log in again!
• Go to Settings > Keyboard, click the + button, select English (United States), and then pick the EurKEY (US) keyboard layout.