# cat /etc/udev/hwdb.d/90-razer-deathadder.hwdb
evdev:input:*v1532p006E*
 KEYBOARD_KEY_90004=btn_middle

# Do note that the hex digits in the device ID *must* be uppercase. In this case there's only one, 'E', where 'v' presumably stands for 'vendor', 'p' for 'product'.

# Do some combination of:
udevadm hwdb --update
systemd-hwdb update
systemctl restart udev