How to use Auth or Flash components inside a cell (login form)?

It’s a login form. It’s almost everywhere (inside default layout). I considered an Element but there is no rendering inside elements. And then I considered calling a controller’s method inside default.ctp and after some search I realized it’s not a good idea.

Then I found out about Cells but I can’t use Auth or Flash or any of the controllers components. So how can I use them? Is it even a good practice? Can I call a controllers method inside a cell to just use the components? Is it a good practice?

Currently the login form is coming from a controller method and has it’s own view:

function login()
if ($this->request->is('post')) {
    $user = $this->Auth->identify();
    if ($user) {
        return $this->redirect($this->Auth->redirectUrl());
    $this->Flash->error('Incorrect login.');

My bad, using a cell for this wasn’t a good idea.
I forgot about create method of Form and how the form tag of html works. I used an Element as:

    <?= $this->Form->create(null, [
        'url' => [
            'controller' => 'Users',
            'action' => 'login'
    ]); ?>
    <?= $this->Form->input('username', ['class' => 'form-control']); ?><
    <?= $this->Form->input('password', ['type' => 'password', 'class' => 'form-control']); ?>
    <?= $this->Form->submit('Login'); ?>
    <?= $this->Form->end(); ?>