type
 TPostMessageData = record
  sString: string;
  sInt: Integer;
 end;
 PPostMessageData = ^TPostMessageData;

Button1Click(Sender: TObject);
var
 CopyData: TPostMessageData;
begin
 CopyData.sString := 'String test';
 CopyData.sInt := 99;

 SendMessage(Handle, WM_COPYDATA, 0, Integer(@CopyData));
end;

procedure TFormReceiver.WMCopyData(var msg: TWMCopyData);
begin
 ShowMessage(PPostMessageData(msg.CopyDataStruct).sString);
end;