A script suspose to work with udev, check out https://benij.ltd/disable-internal-keyboard-when-external-keyboard-is-connected

20-hhkb-events.rules

# When HHKB is connected
ACTION=="add", SUBSYSTEM=="usb", ENV{XAUTHORITY}="/home/user/.Xauthority", ENV{DISPLAY}=":0", ATTRS{idVendor}=="0853", ATTRS{idProduct}=="0100", RUN+="/home/user/bin/hhkb add"
# When HHKB is disconnected.
ACTION=="remove", SUBSYSTEM=="usb", ENV{XAUTHORITY}="/home/user/.Xauthority", ENV{DISPLAY}=":0", ATTRS{idVendor}=="0853", ATTRS{idProduct}=="0100", RUN+="/home/user/bin/hhkb remove"

hhkb.sh

#!/bin/bash

# use $ xinput list to check your keyboard name
deviceName="Asus Keyboard"
# awk is the command to split return the string which is greped with command grep.
# In '{print $4 }', $n is the number of column of the string.
# You should be test it before put this into command down below.
builtInId=$(xinput list | grep $deviceName | awk '{ print $4 }' | grep -Eo '[0-9]{1,2}' | sort | head -n 1)

case $1 in
    add)  
      xinput disable $builtInId
      # dbus-launch is needed for send notification.
      dbus-launch notify-send "HHKB Connected..." \ "Internal keyboard disabled !";;
    remove)  
      xinput enable $builtInId
      dbus-launch notify-send "HHKB Disconnected..." \ "Internal keyboard enabled !";;
esac