1:

  WM_TEST = WM_USER + 5201;					

1:
2:
3:
4:
5:
6:
7:

procedure MessageReceiver(var msg: TMessage); message WM_TEST;
  
procedure TForm1.MessageReceiver(var msg: TMessage);
begin
  if Msg.Msg >= WM_TEST then
  ShowMessage('');
end;					

1:
2:
3:
4:
5:
6:
7:

procedure TForm1.Button1Click(Sender: TObject);
var
  txt: string;
begin
  txt := 'Hello World';
  SendMessage(Form1.Handle, WM_TEST, 0, DWORD(PChar(txt)));
end;					

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:
39:
40:
41:
42:
43:
44:
45:
46:
47:
48:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Forms;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    function ApplicationWndProcHook(var Message: TMessage): Boolean;
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

const
  UM_TEST = WM_USER + 1;

function TForm1.ApplicationWndProcHook(var Message: TMessage): Boolean;
begin
  case Message.Msg of
    UM_TEST:
      OutputDebugString('Message received!');
  end;
  Result := False;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Application.HookMainWindow(ApplicationWndProcHook);
  PostMessage(Application.Handle, UM_TEST, 0, 0);
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  Application.UnhookMainWindow(ApplicationWndProcHook);
end;

end.