Unfortunately, Chrome OS does not allow access to the pen side button by third-party applications. At this time, the pen side button can only be used by the Google Assistant. This means that Squid cannot detect the pen side button on the Pixelbook Pen or any other active pen used on a Chromebook.
We encourage you to let Google know you'd like to be able to use the pen side button in Squid and/or other apps on your Chromebook.
One way you can do this is by submitting a Chrome Feedback Report
and including "#stylusbuttons" in the description.