code: public Response assignMgtKey(String corpNum, MgtKeyType keyType, String itemKey,
                                   String mgtKey, String userID) throws PopbillException {
          if (itemKey == null || itemKey.isEmpty())
              throw new PopbillException(-99999999, "아이템키(ItemKey)가 입력되지 않았습니다.");
  
          if (mgtKey == null || mgtKey.isEmpty())
              throw new PopbillException(-99999999, "문서관리번호(MgtKey)가 입력되지 않았습니다.");
  
          String PostData = "MgtKey=" + mgtKey;
  
          return httppost("/Taxinvoice/" + itemKey + "/" + keyType.name(),
                  corpNum, PostData, userID, "", Response.class, "application/x-www-form-urlencoded; charset=utf-8");
      }
docs: /*
  (non-Javadoc)
  @see com.popbill.api.TaxinvoiceService#assignMgtKey(java.lang.String, com.popbill.api.taxinvoice.MgtKeyType, java.lang.String, java.lang.String, java.lang.String)