How to receive Window-Messages (WM_...) in my serviceapplication?

Is there any possibility do receive WindowMessages (wm_UserChanged-Messages)
in my serviceapplication?

With "allocateHWnd" i installed a WndProc in the service. And with the
api-function "WTSRegisterSessionNotification" i already get
"WM_WTSSESSION_CHANGE"-messages inside this WndProc. But unfortunately no
other WindowMessages, it seems. Is it necessary to register the WndProc in
any way first (eventually with "registerClass" or something)?

Any ideas somewhere?

Thx in advance.

Greetings,
AStringlist