First of all, you are talking about ASP.NET, not ASP. So technically you are in the wrong forum.
ASP.NET *always* assigns a name=
to form fields, including dropdownlist.
So you could simply bring the page up in a browser, click on the VIEW menu, click on the SOURCE menu item, and then look in the HTML that ASP.NET is generating to find the name it uses.
HOWEVER... If you are using Request.Form
to get values from ASP.NET controls, you are really not using ASP.NET the way it was designed to be used.
That shows how you should be capturing "onchange" for asp:dropdownlist.
Generally speaking, if you find yourself using Request.Form a lot, you haven't really bought into the ASP.NET way of handling forms.