BannerBlockExtension.php

<?php
namespace App\Extensions;

use SilverStripe\ORM\DataExtension;

class BannerBlockExtension extends DataExtension 
{

    private static $db = [
        'Subtitle' => 'Varchar(100)'
    ];

    public function updateCMSFields(FieldList $fields) 
    {
        $fields->insertAfter('Title', new TextField('Subtitle'));
    }
}

extensions.yml

SilverStripe\ElementalBannerBlock\Block\BannerBlock:
  extensions:
    - App\Extensions\BannerBlockExtension