code: public void addMessageTransportType(BaseMessage externalTrxMessage)
      {
          String strTransportType = this.getMessageTransportType();
          Record recMessageTransport = this.getRecord(MessageTransportModel.MESSAGE_TRANSPORT_FILE);
          if (recMessageTransport == null)
              recMessageTransport = Record.makeRecordFromClassName(MessageTransportModel.THICK_CLASS, this);
          try {
              recMessageTransport.setKeyArea(MessageTransportModel.CODE_KEY);
              recMessageTransport.getField(MessageTransportModel.CODE).setString(strTransportType);
              if (recMessageTransport.seek(DBConstants.EQUALS))
              {   // Always
                  TrxMessageHeader trxMessageHeader = (TrxMessageHeader)externalTrxMessage.getMessageHeader();
                  if (trxMessageHeader == null)
                  {
                      trxMessageHeader = new TrxMessageHeader(null, null);
                      externalTrxMessage.setMessageHeader(trxMessageHeader);
                  }
                  Map<String,Object> mapTransport = trxMessageHeader.getMessageTransportMap();
                  if (mapTransport == null)
                      mapTransport = new HashMap<String,Object>();
                  mapTransport.put(MessageTransportModel.TRANSPORT_ID_PARAM, recMessageTransport.getCounterField().toString());
                  trxMessageHeader.setMessageTransportMap(mapTransport);
              }
          } catch (DBException ex) {
              ex.printStackTrace();
          }
      }
docs: From this external message, get the message information (in a map).
  @param externalMessage The external message.