Can't grab keyboard from QWindow created with QWindow::fromWinId from HWND pointer

I'm trying to grab the keyboard input on a QWindow created with QWindow::fromWinId from a HWND pointer, in Windows. I simply can't!

The HWND pointer is provided by another application as this windows is supposed to appear as a Tool window of the application. This is what I am doing:

_window = QWindow::fromWinId((WId)ptr);
_window->requestActivate();
_window->setKeyboardGrabEnabled(true);

I have also tried the following without success:

_widget = QWidget::createWindowContainer(_window);
_widget->setFocus();
_widget->grabKeyboard(); 

No matter what I try, the keyboard events are always passed to the host app and I can't use them in text qml input boxes.