I don't know where you are struck up
I used this code
protected void Button1_Click(object sender, EventArgs e)
{
TextBox tt = new TextBox();
form1.Controls.Add(tt);
}
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click"
style="height: 26px" />
</div>
</form>
</body>
</html>