Doing this for a particular order is pretty straightforward. All you need to have is the ID of the coupon code: // This is the ID of the coupon code you want to add to the order. $coupon_id = 1; /** @var \Drupal\commerce_order\Entity\OrderInterface $order */ $order = \Drupal\commerce_order\Entity\Order::load(1); $order->get('coupons')->appendItem($coupon_id); $order->save()...
Jun