I can't see why you couldn't. I guess this is because you tried different things but
That just can't work. It's either
Generally when I want to double check file paths, I use File.openDialog().fsName; Eventually, uou can use $.evalFile but #include should work.
Here is how I do it.
#include "/h/Function List.jsx";
You have to have forward slashes not back slashes, so I copy and paste the path then replace the slashes and get rid of the colon and add one extra slash at the start. I hope this clears it up.
An easy test to see if get the format right is this (and if you are certain that the file or folder exists):