Adding custom fields to Ninja Forms

I'm trying to add a custom field to Ninja Forms (v. 3.3). Can't find a complete example anywhere.

Digging through the code it seems that the filter 'ninja_forms_register_fields' would do the trick but I can't get it to run anywhere.

Here is how to create/add new Ninja Forms field type (keep in mind this code should be moved into separate WordPress plugin).

First we need to hook into ninja_forms_register_fields:

add_filter( 'ninja_forms_register_fields', array($this, 'register_fields'));

Then define method register_fields (inside plugin class):

public function register_fields($actions) {
    $actions['blah'] = new NF_CustomPlugin_Fields_Blah(); 

    return $actions;
}

Last step is to declare NF_CustomPlugin_Fields_Blah class:

class NF_CustomPlugin_Fields_Blah extends NF_Fields_Textbox {
    protected $_name = 'blah';
    protected $_section = 'common'; // section in backend
    protected $_type = 'textbox'; // field type
    protected $_templates = 'textbox'; // template; it's possible to create custom field templates

    public function __construct() {
        parent::__construct();

        $this->_nicename = __( 'Blah Field', 'ninja-forms' );
    }
}