root@asu-hp840:/etc/systemd/system# cat /usr/local/sbin/mapbrightnesskeys.sh
<code> 
#!/bin/sh

setkeycodes e012 122
setkeycodes e017 123
</code>

root@asu-hp840:/etc/systemd/system# cat /etc/systemd/system/mapbrightnesskeys.service
<code>
[Unit]
Description=Map brigtness keys on HP EliteBook 840 G3
Before=graphical.target

[Service]
ExecStart=/usr/local/sbin/mapbrightnesskeys.sh

[Install]
WantedBy=graphical.target
</code>

<code>
systemctl daemon-reload
systemctl enable mapbrightnesskeys
</code>

Then load Xmodmap automatically at login (delay needed for some reason)
cat ~/.xmodmap
<code>
keycode 130 = XF86MonBrightnessDown NoSymbol XF86MonBrightnessDown NoSymbol XF86MonBrightnessDown
keycode 131 = XF86MonBrightnessUp NoSymbol XF86MonBrightnessUp NoSymbol XF86MonBrightnessUp
</code>

<code>
sunkan@asu-hp840:~$ cat ~/.xsessionrc
# Load mapped brigtness keys for HP EliteBook 840 G3
( sleep 5; /usr/bin/xmodmap ~/.xmodmap ) &
</code>