使一个button先执行后台的Click再执行javascript的onclick

HTML:

<script type="text/javascript">
 function openEg();
</script>
<body>
<asp:Button  runat="server" Text="查询" onclick="btnsel_Click" />
</body>

后台代码:

protected void btnsel_Click(object sender, EventArgs e)
        {
//如果有UpdatePanel就用如下代码调用前台js

ScriptManager.RegisterStartupScript(UpdatePanel1,this.Page.GetType(), "", "if(confirm('确认添加么')){document.getElementById('hdf_MC').value='yes'}", true;
//如果没有,写code 
ClientScript.RegisterStartupScript(Page.GetType(), "ww", "if(confirm('确认添加么')){document.getElementById('hdf_MC').value='yes'}", true);
 }