Örnek uygulamayı buradan deneyebilirsiniz. Kaynak kodunu da buradan indirebilirsiniz.

//opener.aspx
protected void Page_Load(object sender, EventArgs e)
{
        Button1.Attributes.Add("onClick", "window.open('child.aspx?io1=TextBox1&io2=TextBox2','Pencerem','width=180,height=120')");
}

.................... 
//child.aspx
<script type="text/javascript" language="javascript">
        function arkayayolla(deger1,deger2,io1,io2)
        {
            window.opener.document.getElementById(io1).value=deger1;
            window.opener.document.getElementById(io2).value=deger2;
        }
 </script>
...................

//child.aspx.cs
protected void Button1_Click(object sender, EventArgs e)
{
        Label1.Text = "<script>arkayayolla('" + TextBox1.Text + "','" + TextBox2.Text + "','" + Request.QueryString["io1"] + "','" +        Request.QueryString["io2"] + "')</script>";
}