ASP.Net Nested GridViews GridView inside GridView Expand First Row On Page Load

This way

<script type="text/javascript">
    $(function () {
        $("[id*=gvCustomers] td").eq(0).closest("tr").after("<tr><td></td><td colspan = '999'>" + $("[id*=pnlOrders]").html() + "</td></tr>")
    });
</script>