private void AddTextBoxEvent(string fieldName, PdfStamper stamper, string script)
    {
        var item = stamper.AcroFields.GetFieldItem(fieldName);
        if (item != null)
        {
            var dict = item.GetWidget(0);
            if (dict != null)
            {
                var aaDict = dict.GetAsDict(PdfName.AA);
                var action = PdfAction.JavaScript(script, stamper.Writer);
                if (aaDict == null) aaDict = new PdfDictionary();
                aaDict.Put(PdfName.F, action);
                dict.Put(PdfName.AA, aaDict);
            }
        }
    }