docs: @Override
      public String getPopUpURL(String CorpNum, MgtKeyType KeyType,
                                String MgtKey, String UserID) throws PopbillException {
          if (KeyType == null)
              throw new PopbillException(-99999999, "관리번호형태가 입력되지 않았습니다.");
          if (MgtKey == null || MgtKey.isEmpty())
              throw new PopbillException(-99999999, "관리번호가 입력되지 않았습니다.");
  
          URLResponse response = httpget("/Taxinvoice/" + KeyType.name() + "/"
                  + MgtKey + "?TG=POPUP", CorpNum, UserID, URLResponse.class);
  
          return response.url;
      }