var selectedOption = $("#DropDownList1 option:selected").text();

$("#DropDownList1").change(function(e) {
      url: "mysite.com/default.aspx?key=" + selectedOption;
      window.location = url;
});

未测试，也不确定是什么事件重新加载页面e。g.（提交或锚）
here的另一个选项（可能更好）

$(document).ready(function() {
  
   var selectedOption = $("#DropDownList1 option:selected").text();

  $("#DropDownList1").change(function() {
    $.ajax({
      type: "POST",
      url: "mysite.com/default.aspx?key=" + selectedOption,
      data: "{}",
      contentType: "application/json; charset=utf-8",
      dataType: "json",
      success: function(msg) {
        // Replace the div's content with the page method's return.
        alert("this worked!");
      }
    });
  });
});

我不确定你想做什么，但我猜jquery是你问题的正确答案
无论如何，这可能会有所帮助：

<script language="javascript" type="text/javascript">
    function xx(e) {
        alert("fired by " + "<%= DropDownList1.UniqueID %>" + "change ");
        __doPostBack("<%= DropDownList1.UniqueID %>", "");
    }
</script>

<asp:DropDownList ID="DropDownList1" runat="server" onchange="xx()">
    <asp:ListItem>q</asp:ListItem>
    <asp:ListItem>w</asp:ListItem>
    <asp:ListItem>e</asp:ListItem>
    <asp:ListItem></asp:ListItem>
</asp:DropDownList>

代码隐藏（VB.NET）

Protected Sub DropDownList1_SelectedIndexChanged(sender As Object, e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged
    MsgBox("Do whatever you want here")
End Sub