js给input赋值，后台获取的问题

JS中获取dropdownlist的值

var ddl = document.getElementById("DropDownList_ParkName")
var index = ddl.selectedIndex;
var Value = ddl.options[index].value;
var Text = ddl.options[index].text;

给隐藏输入框传值

var input = document.getElementsByName("HiddenSite");
input[0].value=Value;

隐藏输入框

<input  id="HiddenSite" type="hidden" name="HiddenSite" runat="server" />

后台获取输入框的值

string siteID= Request.Form["HiddenSite"].Trim();