Using PlaceHolder to create textbox when click on a button

my .aspx page:
<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
<asp:PlaceHolder ID="PlaceHolder2" runat="server"></asp:PlaceHolder>
<asp:PlaceHolder ID="PlaceHolder3" runat="server"></asp:PlaceHolder>
<asp:PlaceHolder ID="myPlaceHolder" runat="server"></asp:PlaceHolder>
<br />
<br />
<asp:Button ID="Button1" runat="server" Text="Add Test" />
<asp:Button ID="Button2" runat="server" Text="Add Using Loop" />
my .aspx.vb page:
 
Partial Class sample
    Inherits System.Web.UI.Page
    Dim tb As New TextBox
    Dim tb2 As New TextBox
    Dim tb3 As New TextBox
    Dim tb4 As New TextBox
...