The following is the only method I have found to call a DialogBox with ASP.NET

I have a server side button that has client side code that runs when it is clicked.

<MainForm.aspx>

Code:
<script...
Sub cmdCust_OnClick
  Dim sCustomers, sOptions
				
  sOptions = "dialogHeight: 325px; dialogWidth: 400px; Center: Yes; Scroll: No; Status: No; Help: No;"
	
  sCustomers = window.showModalDialog("customersframe.htm",,sOptions)
  Document.getElementById("lblInfo").innertext = sCustomers
End Sub
..\script>
The value returned from the dialogbox is written to "lblInfo" which is a server side label.

The Page_Load event contains the following code

Code:
Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
  If (IsPostBack) Then
    Dim sCust As String

    sCust = lblInfo.Text
  end if
End Sub
The lblInfo.Text is empty. Even though it is a server side control, and the text even appears on the form briefly while stepping through the code.

I don't understand why this control contains no data?

I have tried to use a txtbox control as well, with the same result. I also tried to read the value of the lable in the .VB cmdCust.Click(...) event. (By stepping through the code, I realized client side code is run first)

In every case this field is empty and I cannot get the string from the Dialog box to the form. What am I doing wrong??