Hi Amu,
You need to add your text box inside a PlaceHolder control.
take a Placeholder:
<asp:placeholder id="plc" runat="server"></asp:placeholder>
And then add your textbox object to PlaceHolder:
TextBox tx1 = new TextBox();
tx1.Text = "Dynamic Textbox";
plc.Controls.Add(tx1);
I hope it will help you.
Good luck.