It is pretty easy to do it, really.
Go to Options->Controls->CTRL + (or ALT +), write "/shift s" and the assign a wheel button to it.
Adding to that, you could run a whole script instead of just an instruction by doing this instead:
Go to Options->Controls->CTRL + (or ALT +), write "//scriptname" and the assign a wheel button to it.
In LFS folder you'll find a 'scripts' folder, where you can create a script with several instructions as for example
/shift s
/say Sorry guys!