Javascript Code
script type = "text/javascript">
function SetTarget() {
document.forms[0].target = "_blank";
}
/script>
Asp.net Code
asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" OnClientClick = "SetTarget();" /
C#.net Code
protected void Button1_Click(object sender, EventArgs e)
{
Response.Redirect("page2.aspx");
}