To set up the ASP.NET Trace target in the configuration file, put the following in web.nlog file in your web application directory.

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> 
  <targets> 
    <target name="aspnet" xsi:type="ASPNetTrace" layout="${logger} ${message}"/> 
  </targets> 
  <rules> 
    <logger name="*" minlevel="Debug" writeTo="aspnet"/> 
  </rules> 
</nlog> 

This assumes just one target and a single rule. More configuration options are described here.

To configure the target programmatically, put the following piece of code in your Application_OnStart() handler in Global.asax.cs or some other place that gets executed at the very beginning of your code:

using System; 
using System.Web; 
 
using NLog; 
using NLog.Targets; 
 
namespace SomeWebApplication 
{ 
    public class Global : System.Web.HttpApplication 
    { 
        // 
        // this event handler is executed at the very start of the web application 
        // so this is a good place to configure targets programmatically 
        //  
        // alternative you could place this code in a static type constructor 
        // 
        protected void Application_Start(Object sender, EventArgs e) 
        { 
            ASPNetTraceTarget target = new ASPNetTraceTarget(); 
            target.Layout = "${logger} ${message}"; 
 
            NLog.Config.SimpleConfigurator.ConfigureForTargetLogging(target, LogLevel.Debug); 
        } 
    } 
}

Fully working C# project can be found in the Examples/Targets/Configuration API/ASPNetTrace directory along with usage instructions.