How do I create an iTextSharp watermark? I can't get this code to work.

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.IO;
 
using iTextSharp.text;
using iTextSharp.text.pdf;
 
public partial class watermark_again : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
 
    }
    protected void btnInsertWatermark_Click(object sender, EventArgs e)
    {
        MemoryStream strm = new MemoryStream();
        PdfReader pdfReader = new PdfReader(txtFileLocation.Text);
        PdfStamper stamp = new PdfStamper(pdfReader, strm);       
 
        iTextSharp.text.Image img = iTextSharp.text.Image.GetInstance(txtWatermarkLocation.Text);
        img.SetAbsolutePosition(250, 300);
 
        PdfContentByte waterMark = stamp.GetUnderContent(1);
        waterMark.AddImage(img);
        stamp.FormFlattening = true;
        stamp.Close();       
    }
}