Create one and more textarea's by clicking on a button.

Hello there,

I hope some one can help me with a simple function.
I'm new with asp.net and i want to create some textarea's by clicking on a button.

I managed my self to create a one textarea, but i want that he creates more when clicking again on the button.

Lets check out my code atm.
This is my html code:
<div> <form id="form1" runat="server"> <asp:Button ID="add_msg" runat="server" Text="Add Messagebox" OnClick="add_textbox" /> <asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder> </form> </div> </body>
And this is the server side:
TextBox tb; static int i = 0; protected void add_textbox(object ...