IN PARENT PAGE

IN C# CODE 
protected void gvIndent_RowDataBound(object sender, GridViewRowEventArgs e)
        {
            if (e.Row.RowType == DataControlRowType.DataRow)
            {
                e.Row.Attributes[
                "onmouseover"] = "this.style.cursor='hand';this.style.textDecoration='underline';";

                e.Row.Attributes[
                "onmouseout"] = "this.style.textDecoration='none';";

                e.Row.Attributes[
                "onclick"] = ClientScript.GetPostBackClientHyperlink(this.gvIndent, "Select$" + e.Row.RowIndex);

                e.Row.Attributes.Add(
                "onclick", "openPopUp('" + DataBinder.Eval(e.Row.DataItem, "indent_no") + "');");
            }
        }

In Page Load Event 
 string updateValuesScript = @"function updateValues(popupValues)
                    {
                         document.getElementById('ctl00_PlaceHolderLeft_txtIndentNo').value = popupValues[0];
                    }";

            this.ClientScript.RegisterStartupScript(Page.GetType(), "UpdateValues", updateValuesScript.ToString(), true);
 
IN ASPX
    <script type="text/javascript">
        function openPopUp(strUser) {
           
            var popUrl = 'indent_status.aspx?Value='+strUser;
          
            var name = 'popUp';
            var appearence = 'dependent=yes,menubar=no,resizable=yes,' +
                                          'status=no,toolbar=no,titlebar=no,' +
                                          'left=100,top=80';
            var openWindow = window.open(popUrl, name, appearence);
            openWindow.focus();
        }
    </script>


IN CHILD ASPX PAGE

protected void Page_Load(object sender, EventArgs e)
        {

            string updateParentScript = @"function updateParentWindow(ides)
                              {                                                                              
                               var ide=ides;
                               var arrayValues= new Array(ide);
                               window.opener.updateValues(arrayValues);      
                               window.close();
                              }";
            //document.getElementById('lblID.ClientID').value;
            this.ClientScript.RegisterStartupScript(this.GetType(), "UpdateParentWindow", updateParentScript, true);



btnClose.Attributes.Add("onclick", "updateParentWindow(parameter value)");          
        }