Just a simple redirect ASPX page

I'd use a generic handler (ASHX):

<%@ WebHandler Language="C#" Class="Handler" > 
  
using System;  
using System.Web;  
  
public class Handler : IHttpHandler  
{  
    public void ProcessRequest(HttpContext context)  
    {  
        context.Response.RedirectLocation = "<a href="http://new.location.net/optional/path" target="_blank">http://new.location.net/optional/path</a>";  
        context.Response.StatusCode = 301; // 301 = Moved Permanently | 302 = Object moved | 307 = Temporary redirect  
    }  
  
    public bool IsReusable => false;  
}

No need to load session state or go through the whole page life cycle.

Or use Application_BeginRequest on global.asax:

protected void Application_BeginRequest(object sender, EventArgs e)
{
    var context= ((System.Web.HttpApplication) sender).Context;

    if (/* whatever condition */)
    {
        context.Response.RedirectLocation = "<a href="http://new.location.net/optional/path" target="_blank">http://new.location.net/optional/path</a>";  
        context.Response.StatusCode = 301; // 301 = Moved Permanently | 302 = Object moved | 307 = Temporary redirect
        context.Response.End();
    }
}