Set dropdownlist selected value to hidden field on onchange event of Dropdownlist

 <script type="text/javascript" language="javascript">
        function getfacid() {
            var facid = $('#<%=ddl_facility.ClientID%>').val()
            $("#<%= hdnfacid.ClientID %>").val(facid);
            var hval = document.getElementById('<%=hdnfacid.ClientID %>').value;


        }
    </script>

<input id="hdnfacid" runat="server" type="hidden" />

  <asp:DropDownList runat="server" ID="ddl_facility" Width="170px" onchange="getfacid()">
                                    </asp:DropDownList>
                                 

Put Usercontrol ASCX in ASPX page

<%@ Register Src="USRSupplierDetails.ascx" TagName="USRSupplierDetails" TagPrefix="uc1" %>

 <uc1:usrsupplierdetails id="USRSupplierDetails1" runat="server" />