Cannot add button in updateOperation

I need some help.

I'm trying to add a "preview" button in the updateOperation, so I have the following content:

public function setupUpdateOperation()
{
    $this->crud->setUpdateContentClass('col-md-12');
    $this->setupCreateOperation('update');
    $this->crud->addButtonFromView('line', 'preview', 'preview', 'beginning');
}

but the button doesn't appear at all, I have also created the blade file within this directory: vendor/backpack/crud/buttons/preview.blade.php, which have this content:

<a href="/intranet/{{ $entry->slug }}?preview=1">
    <i class='fa fa-list'></i>Preview
</a>

Essentially, I need to add a button near "Annulla":

<img src="https://i.stack.imgur.com/nGafO.jpg">

There is no built-in way to add buttons after the "Cancel" button. But you can:

(A) Add a widget at the bottom of the form, with your own custom JS, that adds that button to the right container. You can use the script widget for that, and you should push the widget to the after_content widget section, which is the default, so it will be:

Widget::add()->type('script')->content('path/to/your/public/script.js');

(B) Publish Backpack's form_save_buttons blade file to your project, and add that button there. That will make the change for ALL forms, for ALL entities. But this will also mean you no longer get updates for that file, from Backpack. You've overridden it. You can do that using:

php artisan backpack:publish crud/inc/form_save_buttons

I heavily recommend you do Option A, because that one doesn't have any downsides.