SHTML is built SERVER SIDE--- what is sent to the client (user's browser) is flat HTML no different than if you cut/paste that file in to replace your <!--#include>
NO difference! Ok, try this. Make a page using your SHTML include... put it on your server, load it in your browser, and hit "view source".
Though for what you want to do with it -- I'd either switch to a real scripting language to change a hidden field using a variable or parameter to a function, or use multiple includes to build the form.