Passing session variable between two forms

I've got two forms. The parent opens a kind of open file dialog box in the form of a web page. The child form consists of a listbox which gets populated with filenames, and a command button. I would like to pass the selected value back to the parent form. This is what I have so far:

VB Code:

'Parent Form
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        RegisterStartupScript("ClientSideScript", "<script language=javascript>window.open('OpenFile.aspx', null, 'location=no;status=no;addressbar=no;toolbar=no,height=350,width=250')</script>")
    End Sub
 
    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Try
            TextBox1.Text = Session("name").ToString()
        Catch oEX As Exception
            Console.WriteLine(oEX.ToString)
        End Try
    End Sub

VB Code:

'Child Form
    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim sfile As String = Dir("C:\Temp\*.xml", FileAttribute.Normal)
        While sfile <> Nothing
            sfile = sfile.Substring(0, sfile.IndexOf("_"c)) & ", " & sfile.Substring(sfile.IndexOf("_"c) + 1)
            sfile = sfile.Substring(0, sfile.Length - 4).ToUpper
            ListBox1.Items.Add(sfile)
            sfile = Dir()
        End While
 
    End Sub
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Session("name") = ListBox1.SelectedValue
        Server.Transfer("webform1.aspx")
        RegisterStartupScript("ClientSideScript", "<script language=javascript>window.close();</script>")
 
    End Sub

The problem is, the parent form reopens in the child form's browser after I pass the variable. I would really like the child form to close, and have the original parent form handle the variable.

I was able to prevent the parent page from loading to the child browser by taking out the Server.Transfer line.
My new question is, how can I make the parent window wait for the child window to close before declaring a string variable set to the session variable's value?

Here's the code I used to force the postback.

Code:

---

        RegisterStartupScript("ClientSideScript", "<script language=javascript>window.opener.document.forms[0].submit();window.close();</script>")

---