To set up the target in the configuration file, use the following syntax:

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> 
  <targets> 
    <target name="control" xsi:type="FormControl" append="true" 
            controlName="textBox1" formName="Form1"/> 
  </targets> 
  <rules> 
    <logger name="*" minlevel="Debug" writeTo="control"/> 
  </rules> 
</nlog> 

The result is:

To set up the log target programmatically similar to above use code like this:

using System; 
using System.Text; 
using System.Windows.Forms; 
using NLog; 
using NLog.Targets; 
 
namespace RichTextBox2 
{ 
    public partial class Form1 : Form 
    { 
        public Form1() 
        { 
            InitializeComponent(); 
        } 
 
        private void Form1_Load(object sender, EventArgs e) 
        { 
 
            FormControlTarget target = new FormControlTarget(); 
            target.Layout = "${date:format=HH\\:MM\\:ss} ${logger} ${message}"; 
            target.ControlName = "textBox1"; 
            target.FormName = "Form1"; 
            target.Append = true; 
 
            NLog.Config.SimpleConfigurator.ConfigureForTargetLogging(target, LogLevel.Trace); 
 
            Logger logger = LogManager.GetLogger("Example"); 
            logger.Trace("trace log message, "); 
            logger.Debug("debug log message, "); 
            logger.Info("info log message, "); 
            logger.Warn("warn log message, "); 
            logger.Error("error log message, "); 
            logger.Fatal("fatal log message"); 
        } 
    } 
}