This stuff here: onChange="enableIt(this,document.sipp.proyektemp); gatherInfoProp(document.sipp.fiscal, this, 'thn=','&kdprop='); getName(this,document.sipp.nmpropinsi); is JS which runs three different functions when something is selected in those menus.

So, figure out what's actually happening, is the usual advice.

My preferred way to do this is to use the Live HTTP Headers Add-on for Firefox. No matter what the Javascript does, sooner or later, the browser retrieves a page, from a URL, via HTTP, and once you can figure out what that URL is, you'll be able to write your script.