Catch a variable posted to an .ashx Generic Handler that is appended as a URL line variable.

<%@ WebHandler Language="C#" Class="Endpoint" %>
using System.Web;
public class Endpoint : IHttpHandler {
   public void ProcessRequest (HttpContext context)
   {
      var whatever = context.Request["Whatever"];
   }
   
   public bool IsReusable {
      get {
         return false;
      }
   }
}