You can use Rout redirecting.
protected void btnNewEntry_Click(object sender, EventArgs e)
{
Response.RedirectToRoute("CMS_1");
}
which requires to define your routing logic in Global.asax file that could be like that:
routes.MapPageRoute("CMS_1", "CMS_1", "~/CMS_1.aspx");
where any request by CMS_1 pattern in application scope will be redirecting to CMS_1.aspx, but in URL shows like www.yoursite.com/CMS_1