bloomberg API: field not permitted to datafeed users

I use the bloomberg API(c++) to do a project. I have already been able to send a request with an identity. For example, I send a request like this:

    Request request = session.createRequest("ReferenceDataRequest");
    request.append("securities", "IBM US Equity");
    request.append("fields", "PX_LAST");
    session.sendRequest(request, identity, CorrelationId(20));

After sending this request, I get a response. But when I release the response, I always get an exception: "NO_AUTH, field not permitted to datafeed users". the function to deal with the response:

void handleResponseEvent(Event e, Identity identity)
{
    MessageIterator msgIter(e);
    while (msgIter.next())
    {
        Message message = msgIter.message();
        message.print(cout);
    }
}

I know that this is not correct because i didn't use the "identity". I think that's why I got an exception "NO_AUTH". Could you please tell me how to release the response with the identity?