function HookTrayProc(code: integer; wParam: integer; lParam: integer): integer; stdcall;
begin
 if code = HC_ACTION then
   if TMsg(Pointer(lParam)^).message = WM_DROPFILES then
     SendMessage(FindWindow('TfrmMain', nil), WM_DROPFILES, TMsg(Pointer(lParam)^).wParam, TMsg(Pointer(lParam)^).lParam);
 Result := CallNextHookEx(HookHandle, Code, wParam, lParam)
end;

...

 ResultHook:=SetWindowsHookEx(WH_MOUSE, @HookTrayProc, HInstance, GetWindowThreadProcessId(FindWindow('Shell_TrayWnd', nil)));

function HookTrayProc(code: integer; awParam: integer; alParam: integer): integer; stdcall;
begin
if code = HC_ACTION then
  with PMOUSEHOOKSTRUCT(alParam)^ do
    Application.MainForm.Caption :=  Format('%d %d', [pt.x, pt.y]);
Result := CallNextHookEx(0, Code, awParam, alParam)
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
 SetWindowsHookEx(WH_MOUSE, HookTrayProc, HInstance, GetCurrentThreadId);
end;