<%@ Page Language="C#" %>
<script runat="server">

    protected void Page_Load(object sender, EventArgs e)
    {        
	TextBox1.Focus();
    }

    
</script>
<html>
	<head runat="server">
		<title>Definindo o foco</title>
	</head>
	<body>
		<form id="form1" runat="server">
			<div>
				<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
				<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
			</div>
		</form>
	</body>
</html>
