Identify stylus barrel button (win32)

Question

I am creating an application that uses the tablet API for windows and so far I was able to capture the input of the stylus succesfully. I am able to handle the movement, stylus down and up events and pressure. What I am still missing is the ability to identify the barrel button pressed. While I am capturing the StylusButtonDown event, this is fired also when the stylus is down on the tablet.

For reference here is my (ideal) implementation:

STDMETHOD(StylusButtonDown)(IRealTimeStylus* rts, STYLUS_ID id, const GUID* guid, POINT* pt)
{
  if (BARREL_BUTTON_ID == *guid) {
    s_buttonDown = true;

    // DEBUG
    std::cout << "[StylusButtonDown]" << std::endl;
    // DEBUG
  }
  return S_OK;
}

The problem I have is to fill in the BARREL_BUTTON_ID constant. Any expert with an idea? Thanks a lot!