unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs;

const
   MY_MESSAGE = WM_USER + 42;

type
  TForm1 = class(TForm)


  procedure MessageReceiver(var msg: TMessage); message MY_MESSAGE;

  private

    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.MessageReceiver(var msg: TMessage);
var
 txt: PChar;
begin
  //Получение сообщения
  txt := PChar(msg.lParam);
  msg.Result := 1;
  ShowMessage(txt);
end;

end.

var
  h: HWND;
  txt: string;
begin
  txt := '123';
  var winTitle := '1212';
  h:=FindWindow(nil, PWideChar(winTitle));
  if h <> 0
    then caption := 'Нашел ' + winTitle
    else caption := 'НЕТ ' + winTitle;

  SendMessage(h, MY_MESSAGE, 0, DWORD(PChar(txt)));

var
 aCopyData: TCopyDataStruct;
 hTargetWnd: HWND;
begin
 with aCopyData do
 begin
   dwData := 0;
   cbData := StrLen(PChar(edtText.text)) + 1;
   lpData := PChar(edtText.text)
 end;
 // Search window by the window title
// Fenster anhand des Titelzeilentext suchen
//hTargetWnd := FindWindowEx(0, 0, nil, PChar('WM_COPYDATA-Receiver'));
hTargetWnd := FindWindowEx(0, 0, nil, PChar('1212'));
 if hTargetWnd <> 0 then
   SendMessage(hTargetWnd, WM_COPYDATA, Longint(Handle), Longint(@aCopyData))
 else
   ShowMessage('No Recipient found!');

type
  TForm1 = class(TForm)
    lbl1: TLabel;

  procedure WMCopyData(var Msg: TWMCopyData); message WM_COPYDATA;

  procedure MessageReceiver(var msg: TMessage); message MY_MESSAGE;

  private

    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.WMCopyData(var Msg: TWMCopyData);
var
 sText: array[0..99] of Char;
begin
   // generate text from parameter
  // anzuzeigenden Text aus den Parametern generieren
  StrLCopy(sText, Msg.CopyDataStruct.lpData, Msg.CopyDataStruct.cbData);
   // write received text
  // Empfangenen Text ausgeben
  lbl1.Caption := sText;
end;

cbData := StrLen(PChar(edtText.text)) + 1;

cbData := (Length(edtText.text) + 1) * Sizeof(Char);