code: public static PdfAppearance createAppearance(PdfWriter writer, float width, float height) {
          return createAppearance(writer, width, height, null);
      }
docs: Creates a new appearance to be used with form fields.
  
  @param writer the PdfWriter to use
  @param width the bounding box width
  @param height the bounding box height
  @return the appearance created