You would need to set that Session variable somewhere first before you could use it. It sounds like you might need to set that session variable on the page specified as the form action where the value is entered.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.