one more way
Declare JavaScript block in your content page like below
<%@ Page Language="C#" MasterPageFile="~/Site1.Master" AutoEventWireup="true" CodeBehind="WebForm2.aspx.cs" Inherits="WebApplication4.WebForm2" Title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
<asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>
<asp:TextBox ID="TextBox4" runat="server"></asp:TextBox>
<script>
var controlId_focus= '<%=controlIdForFocus%>';
function CallSetFocus(){
if(controlId_focus !="" && document.getElementById(controlId_focus)!=null) {
document.getElementById(controlId_focus).focus();
}
}
setTimeout('CallSetFocus()',200);
</script>
</asp:Content>
Now in your code behind file declare protected string "controlIdForFocus" and assign ClientID of a control where you want focus like below code snippet
public partial class WebForm2 : System.Web.UI.Page
{
protected string controlIdForFocus = "";
protected void Page_Load(object sender, EventArgs e)
{
controlIdForFocus = TextBox3.ClientID;
}
}