Adding foreign key to existing table using Migrations

PHP Code:

<?php namespace App\Database\Migrations;

use CodeIgniter\Database\Migration;

class AddIduserProduk extends Migration
{
    public function up()
    {
        $fields = [
            'id_user' => [
                'type'      => 'INT',
                'constraint'=> 11,
                'unsigned'  => TRUE,
                'after'         => 'id_kategori',
                'null'            => TRUE
            ],
            'CONSTRAINT produk_ibfk_1 FOREIGN KEY(`id_user`) REFERENCES `users`(`id_user`)'
        ];
        $this->forge->addColumn('product_table', $fields);
    }

    //--------------------------------------------------------------------

    public function down()
    {
        $this->forge->dropForeignKey('produk','produk_ibfk_1');
        $this->forge->dropColumn('product_table', 'id_user');
    }
} 


Its work for me in file app/Database/Migrations/2020-12-31-034310_add_iduser_produk.php