I am using this example code I found online for iTextSharp:
PdfReader template = new PdfReader(“RightsRespPDFJS.pdf”);
XfdfReader xfdfReader = new XfdfReader(“xfdf.xml”);
PdfStamper stamper = new PdfStamper(template, new FileStream(“RightsRespMerged.pdf”, FileMode.Create));
stamper.AcroFields.SetFields(xfdfReader);
stamper.Writer.CloseStream = false;
stamper.Close();