How to add another text box programatically on a textchanged evnt of a text box in a place holder

[Rajesh]

Please give a solution for me for the following code. here i have a text box already after that when a text box value is changed and a new textbox will come in the place holder with postback property=true. After that when iam typing a value in that textbox (new one) and pressing the tab then the form will postback. but me want a new text box will generate there as the first text box textchanged event. how can me resolve the prob. Please help me

test.aspx
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<%@ Page Language="vb" AutoEventWireup="false" Codebehind="test.aspx.vb" Inherits="partyroute.test"%>

cellPadding="1" width="300" border="1">

***************************
test.vb
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Public Class test
Inherits System.Web.UI.Page

Dim k As Integer = 2

#Region " Web Form Designer Generated Code "

'This call is required by the Web Form Designer.
Private Sub InitializeComponent()

End Sub
Protected WithEvents a1 As System.Web.UI.WebControls.TextBox
Protected WithEvents ph As System.Web.UI.WebControls.PlaceHolder
Protected WithEvents lb As System.Web.UI.WebControls.Label

'NOTE: The following placeholder declaration is required by the Web Form Designer.
'Do not delete or move it.
Private designerPlaceholderDeclaration As System.Object

Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
'CODEGEN: This method call is required by the Web Form Designer
'Do not modify it using the code editor.
InitializeComponent()
End Sub

#End Region

Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

If Not IsPostBack Then
ViewState("prd") = 1
lb.Text = ViewState("prd")
Else
If ViewState("prd") > 1 Then
For k = 2 To ViewState("prd")
addfds(k)
lb.Text = "hai"
Next
End If
'lb.Text = ViewState("prd")
End If
End Sub

Sub addfds(ByVal s As String)
Dim txt As TextBox

txt = New TextBox
txt.ID = "a" & s
txt.AutoPostBack = True
'txt.Text = "a" & s
ph.Controls.Add(txt)
ph.Controls.Add(New LiteralControl("
"))
End Sub

Private Sub a1_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles a1.TextChanged
ViewState("prd") = ViewState("prd") + 1
addfds(ViewState("prd"))
End Sub
End Class