Configure your rector.php:

<?php

use RectorLaravel\Rector\Coalesce\ApplyDefaultInsteadOfNullCoalesceRector;
use RectorLaravel\ValueObject\ApplyDefaultInsteadOfNullCoalesce;
use Rector\Config\RectorConfig;

return RectorConfig::configure()
    ->withConfiguredRule(ApplyDefaultInsteadOfNullCoalesceRector::class, [
        ...self::defaultLaravelMethods(),
        new ApplyDefaultInsteadOfNullCoalesce('custom_helper'),
    ]);