procedure ReceiveMessage (var Msg: TMessage);
message WM_COPYDATA;
...
procedure TFormReceive.ReceiveMessage;
var
  pcd: PCopyDataStruct;
begin
  pcd := PCopyDataStruct(Msg.LParam);
  Caption := PChar(pcd.lpData);
end;

procedure TFormXXX.Button1Click(Sender: TObject);
var
  cd: TCopyDataStruct;
begin
  cd.cbData := Length(Edit1.Text) + 1;
  cd.lpData := PChar(Edit1.Text);
  SendMessage(FindWindow('TFormReceive', nil), WM_COPYDATA, 0, LParam(@cd));
end;