Open new Window from Server side without Page Refresh in ASP.Net

Add the Button control inside the UpdatePanel.

Please refer this code.

HTML

<asp:ScriptManager runat="server" />
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
        <asp:Button ID="Button1" Text="Redirect" runat="server" OnClick="Redirect" />
    </ContentTemplate>
</asp:UpdatePanel>

 C#

protected void Redirect(object sender, EventArgs e)
{
    ScriptManager.RegisterClientScriptBlock(sender as Control, this.GetType(), "alert", "window.open('TestPage2.aspx')", true);
}