How to customize the M code to be recognized by linuxcnc

I want to redefine an M code to control the galvanometer, but I don’t know how to define it so that lnuxcnc can run

Have a look at the docs:

linuxcnc.org/docs/html/remap/remap.html#_configuring_remapping

Is it a python remap you are trying to get running?

I would normally expect a galvanometer to be operated by G1 X Y or G0 X Y commands.
Why do you need a custom code?