How to add digital sigature in exsiting signature field in the document?

 
//Loads the Certificate
PdfCertificate pdfCert = new PdfCertificate(@"../../Data/PDF.pfx", "syncfusion");
 
//Load an existing Pdf document
PdfLoadedDocument doc = new PdfLoadedDocument("../../Data/emptypdf_modified.pdf");
 
//Get the signature field to be digitally signed
PdfLoadedSignatureField f = doc.Form.Fields["Firma1"] as PdfLoadedSignatureField;
 
// Pass the signature field and certificate information in PdfSignature class, this  will digitally sign the existing signature field
PdfSignature signature = new  PdfSignature(doc, f.Page, pdfCert, f.Name, f);
 
//Save the document and dispose it.
doc.Save("signed1.pdf");
doc.Close(true);