iTextSharp

I need to change/add the submit action of button of a PDF File. I end up
using the FdfWriter,
/*-----------------------------------*/
PdfReader pdfFile = new PdfReader(pdfFilePath);
PdfStamper stamper = new PdfStamper(pdfFile, newPDF, '\0', true);
FdfWriter fdf = new FdfWriter();
fdf.SetFieldAsString("fieldID", "FDF Barney");

fdf.SetFieldAsAction("myButtonID",
PdfAction.CreateSubmitForm(@"https://mysite/test.aspx",null,PdfAction.SUBMI
T_HTML_FORMAT ));
fdf.WriteTo(newFDF);

FdfReader fdfReader = new FdfReader(newFDF);
stamper.AcroFields.SetFields(fdfReader);
/*-----------------------------------*/

OK Up to here my code works(no questions allowed to my fdf merge), it sets
all the form fields values, but it does not change or adds any action to my
selected field.

How can I change the submit action of my button inside my pdf?

Thanks,