I have a Logitech G15 keyboard which has an LCD screen built into the top of it. Under Ubuntu, getting it to work is a simple matter of running
apt install g15daemon
Under Manjaro, it's a bit different. I can install the g15daemon but it won't start automatically without requring the root password. What we need to do is use a tool called visudo. This is what I did.
Then go to the bottom of the file, press o to insert and put:
ALL ALL= NOPASSWD: /usr/sbin/g15daemon
Press escape, then :x to save and quit.
Now we enable it as a service in systemctl
sudo systemctl enable g15daemon
And test it by running
sudo systemctl start g15daemon
The LCD on the keyboard now shows me the time and date. To get this to start when I log in, because I'm using Manjaro XFCE I will open "Session and Startup", go to the "Application Autostart" tab and create a new entry with "sudo systemctl start g15daemon" as the command to run, giving it a suitable name.
After a reboot to test it, it's all working!
I originally followed this post on the Manjaro forums with no success, I found my way to be much cleaner.