How do I pass value from child window to parent window?

Question

I have an html page (parent window) and an aspx page (child page). I have the values in lblNames.text in the child page. How do I pass the values in label (lblNames) from the child window to the textarea in the parent window?

parent window:

<script type="text/javascript" language="JavaScript">
function showPopup(){
window.showModalDialog('http://something.com/MultiTest.aspx',window,'center:yes');
}
</script>
<form action="something" method="post" name="name"id="form1">
<textarea id="Notes" name="Notes" rows="5" cols="36" style="display: none;"></textarea>

Child window:

Protected Sub btnUpload_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnUpload.Click
       Try
            Dim hfc As HttpFileCollection = Request.Files
            lblNames.Text = String.Empty
            For i As Integer = 0 To hfc.Count - 1
                Dim hpf As HttpPostedFile = hfc(i)
                If hpf.ContentLength > 0 Then
                    hpf.SaveAs(Server.MapPath("/servename/directory") & "\" & System.IO.Path.GetFileName(hpf.FileName))
                    Label1.Text += "<b>File: </b>" & hpf.FileName & "     " & "    Uploaded Successfully! <br/>"
                    lblNames.Text += hpf.FileName & ", "
                End If
            Next i
        Catch ex As Exception
        End Try
End Sub