code: public PdfFormField addSignature(String name, int page, float llx, float lly, float urx, float ury) {
          PdfAcroForm acroForm = stamper.getAcroForm();
          PdfFormField signature = PdfFormField.createSignature(stamper);
          acroForm.setSignatureParams(signature, name, llx, lly, urx, ury);
          acroForm.drawSignatureAppearences(signature, llx, lly, urx, ury);
          addAnnotation(signature, page);
          return signature;
      }
docs: Adds an empty signature.
  @param name	the name of the signature
  @param page	the page number
  @param llx	lower left x coordinate of the signature's position
  @param lly	lower left y coordinate of the signature's position
  @param urx	upper right x coordinate of the signature's position
  @param ury	upper right y coordinate of the signature's position
  @return	a signature form field
  @since	2.1.4