Posting Registered message to another process?

I want to communicate between two SDI processes with something like:
PostMessage(HWND_BROADCAST, gMyRegMsgId, 123, 321);

The "Poster" and "Postee" both have calls in OnInitialUpdate for:
  gMyRegMsgId=
::RegisterWindowMessage("BB_FIND_8DB23EEF_B705_44BE_A58A_EBA6481A5038");

***** Poster code ****
void CPosterRegMsgView::OnClickPostButton()
{
  BOOL flag = ::PostMessage(HWND_BROADCAST, gBbFindMsgId, 1234, 4321);
}

***** Postee code ****
BEGIN_MESSAGE_MAP(CPosteeRegMsgView, CFormView)
 //{{AFX_MSG_MAP(CPosteeRegMsgView)
  ON_REGISTERED_MESSAGE (gMyRegMsgId, OnBbFindMsgId)
 //}}AFX_MSG_MAP
END_MESSAGE_MAP()

LRESULT CPosteeRegMsgView::OnBbFindMsgId(WPARAM wParam, LPARAM lParam)
{
  CString msg;
  msg.Format("W: %d L: %d", wParam, lParam);
  ::MessageBox(NULL, msg, "CPosteeRegMsgView", MB_OK);
  return 0;
}

***********

With the above, I can step through each of the programs and check that
gMyRegMsgId is the same. However, when I click on the button of the "Poster"
program, nothing seems to be received by the "Postee" program. I'm expecting
the MessageBox to show up, confirming the desired communication.

What am I doing wrong? Is the HWND_BROADCAST argument to PostMessage
incorrect?