codice:
<!DOCTYPE html>
    <script type="text/javascript">
        function BlankTab() {
            window.open('http://www.html.it');
        }
    </script>


</head>
<body>
    <form id="form1" runat="server">
        <div>
            <asp:ImageButton runat="server" ID="ibTest" OnClick="ibTest_Click" OnClientClick="BlankTab();" ImageUrl="immagini/button.jpg" />
        </div>
    </form>
</body>
</html>