<input type="button" runat="server" id="Button2" onserverclick="Button2_Click" value="Button2"/>

asp.net中想要使<input type="button"调用后台的的方法，只需在标签中添加runat="server"并将onclick改为onserverclick，然后写后台方法的方法名即可onserverclick="Button2_Click" 

<input type="button" runat="server" id="Button2" onserverclick="Button2_Click" value="Button2"/>

protected void Button2_Click(object sender, EventArgs e)
 {

}