{Code for Unit1}

const
  UM_MyGlobalMessage = WM_USER + 1;

type
  TForm1 = class(TForm)
    Label1: TLabel;
    Button1: TButton;
    procedure FormShow(Sender: TObject);
    procedure Button1Click(Sender: TObject);
    { Private declarations }
  private
    procedure UMMyGlobalMessage(var AMessage: TMessage); message
      UM_MyGlobalMessage;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

uses Unit2;

procedure TForm1.FormShow(Sender: TObject);
begin
  Form2.Show;
end;

procedure TForm1.UMMyGlobalMessage(var AMessage: TMessage);
begin
  Label1.Left := AMessage.WParam;
  Label1.Top := AMessage.LParam;
  Form1.Caption := 'Got It!';
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  f: integer;
begin
  for f := 0 to Screen.FormCount - 1 do
    Screen.Forms[f].Perform(UM_MyGlobalMessage, 42, 42);
end;

{Code for Unit2}

const
  UM_MyGlobalMessage = WM_USER + 1;

type
  TForm2 = class(TForm)
    Label1: TLabel;
  private
    { Private declarations }
    procedure UMMyGlobalMessage(var AMessage: TMessage); message
      UM_MyGlobalMessage;
  public
    { Public declarations }
  end;

var
  Form2: TForm2;

implementation

{$R *.DFM}

procedure TForm2.UMMyGlobalMessage(var AMessage: TMessage);
begin
  Label1.Left := AMessage.WParam;
  Label1.Top := AMessage.LParam;
  Form2.Caption := 'Got It!';
end;