Please follow this step:
1. Add this link in Head tag:
<script src="//code.jquery.com/jquery-1.10.2.js"></script>
2. Assume your HTML like this:
<asp:dropdownlist id="ComboBox1" runat="server">
<asp:listitem value="1">Hanoi</asp:listitem>
<asp:listitem value="2">HCM City</asp:listitem>
</asp:dropdownlist>
<a href="User/SubCat.aspx?cid=wer">Redirect</a>
3. The script to get value in Combobox 1 then append param to href of the <a> tag below:
<script>
$(document).ready(function () {
var _cityId = $('#<%= ComboBox1.ClientID %>').val();
$("a").each(function () {
$(this).prop('href', $(this).attr('href') + '&cityid=' + _cityId);
});
});
</script>