'providers' => [
    // ...
    Alfa6661\AutoNumber\AutoNumberServiceProvider::class,
],

use Alfa6661\AutoNumber\AutoNumberTrait;
    
class Order extends Model
{
    use AutoNumberTrait;
    
    /**
     * Return the autonumber configuration array for this model.
     *
     * @return array
     */
    public function getAutoNumberOptions()
    {
        return [
            'order_number' => [
                'format' => 'SO.?', // autonumber format. '?' will be replaced with the generated number.
                'length' => 5 // The number of digits in an autonumber
            ]
        ];
    }

}

public function getAutoNumberOptions()
{
    return [
        'order_number' => [
            'format' => function () {
                return 'SO/' . date('Ymd') . '/?'; // autonumber format. '?' will be replaced with the generated number.
            },
            'length' => 5 // The number of digits in the autonumber
        ]
    ];
}

$order = Order::create([
    'customer' => 'Mr. X',
]);

echo $order->order_number;

// SO/20170803/00001