How do I apply a coupon at checkout?

Question

1C-Bitrix

there is a certain hardcode that turns the data from the form into an order for the goods.
Everything works and everyone is happy with everything.
You need to add the ability to add a coupon to it.
I added the following:

//купон
        $coupon = "testtest";
        \Bitrix\Sale\DiscountCouponsManager::add($coupon);
        $oBasket = \Bitrix\Sale\Basket::loadItemsForFUser(
            \Bitrix\Sale\Fuser::getId(),
            \Bitrix\Main\Context::getCurrent()->getSite()
        );
        $discounts = $order->getDiscount();
        $discounts->calculate();
        // // купон

I can't figure out why it doesn't add.

Here is the whole code

$formId = 10;
        if ($WEB_FORM_ID != $formId) return;
        $orderIdStr = trim($arrVALUES['form_hidden_52']);
        if (!$orderIdStr) return;


        $matches = null;
        $pattern = '/\\[(\\d+)\\].*/';
        $res = preg_match($pattern, $orderIdStr, $matches);

        if (!$matches[1]) return;

        \Bitrix\Main\Loader::includeModule('sale');
        \Bitrix\Main\Loader::includeModule('catalog');
        \Bitrix\Main\Loader::includeModule('iblock');


        $productId = $matches[1];
        $name = $arrVALUES['form_text_49'];
        $phone = $arrVALUES['form_text_50'];
        $email = $arrVALUES['form_email_51'];
        $location = $arrVALUES['LOCATION'];
        $location_id = $arrVALUES['LOCATION_ID'];

        global $siteId;

        $prod = \Bitrix\Iblock\ElementTable::getList(array(
            'filter' => ['ID' => $productId],
            'select' => ['NAME'],
        ))->Fetch();

        $product = [
            'NAME' => $prod['NAME'],
            'CURRENCY' => 'RUB',
            'QUANTITY' => 1,
            'PRODUCT_PROVIDER_CLASS' => '\Bitrix\Catalog\Product\CatalogProvider',
        ];

        $basket = Bitrix\Sale\Basket::create($siteId);

        $item = $basket->createItem("catalog", $productId);
        $item->setFields($product);

        $currencyCode = \Bitrix\Currency\CurrencyManager::getBaseCurrency();

        if ($arFields['USER_AUTH'] == "'Y'") {
            $registeredUserID = $arFields['USER_ID'];
        } else {
            $registeredUserID = Site::createUser([
                'NAME' => $name,
                'PERSONAL_PHONE' => $phone,
                'EMAIL' => $email,
            ]);
        }
        if (!$registeredUserID) {
            array("STATUS" => "ERROR", "MESSAGE" => "Ошибка при оформлении заказа");
        }
        $order = \Bitrix\Sale\Order::create($siteId, $registeredUserID);


        $order->setPersonTypeId(1);
        $order->setBasket($basket);

        $order->setField('CURRENCY', $currencyCode);
        $order->setField('STATUS_ID', 'PR');  //предзаказ
        $order->setField('COMMENTS', 'ГОРОД ЗАКАЗА: (' . $location_id . ")" . $location);

        // Устанавливаем свойства
        $info = ['YOUR_FAMILY' => $name, 'YOUR_PHONE' => $phone, 'YOUR_EMAIL' => $email, 'LOCATION' => $location];
        $c = $order->getPropertyCollection();
        foreach ($c->getGroups() as $g) foreach ($c->getGroupProperties($g['ID']) as $p) {
            $a = $p->getProperty()['CODE'];
            if (isset($info[$a])) $p->setValue($info[$a]);
        }

        $shipmentCollection = $order->getShipmentCollection();
        $shipment = $shipmentCollection->createItem(
            Bitrix\Sale\Delivery\Services\Manager::getObjectById(1)
        );


        $shipmentItemCollection = $shipment->getShipmentItemCollection();
        foreach ($basket as $basketItem) {
            $item = $shipmentItemCollection->createItem($basketItem);
            $item->setQuantity($basketItem->getQuantity());
        }


        $paymentCollection = $order->getPaymentCollection();
        $payment = $paymentCollection->createItem(
            Bitrix\Sale\PaySystem\Manager::getObjectById(6) //Банковской картой на сайте
        );

        $payment->setField("SUM", $order->getPrice());
        $payment->setField("CURRENCY", $order->getCurrency());

        //купон
        $coupon = "testtest";
        \Bitrix\Sale\DiscountCouponsManager::add($coupon);
        $oBasket = \Bitrix\Sale\Basket::loadItemsForFUser(
            \Bitrix\Sale\Fuser::getId(),
            \Bitrix\Main\Context::getCurrent()->getSite()
        );
        $discounts = $order->getDiscount();
        $discounts->calculate();
        // // купон


        $result = $order->save();
        global $APPLICATION;
        $APPLICATION->RestartBuffer();
        if ($result->getId()) {
            $accountNum = $order->getField("ACCOUNT_NUMBER");
            echo "Ваш заказ {$accountNum} оформлен";
        } else {
            echo 'Ошибка создания предзаказа';
        }
        die();

Answer 1

Here is the coupon block earned

//купон

        \Bitrix\Sale\DiscountCouponsManager::add($coupon);

        $discounts = $order->getDiscount();
        $discounts->setOrderRefresh(true);
        $discounts->calculate();
        $basket->refreshData(["PRICE", "COUPONS"]);
        $order->doFinalAction(true);

        // // купон

Answer 2

See an example of adding https://dev.1c-bitrix.ru/api_d7/bitrix/sale/classe... , I think just for your task.
You are using a coupon, without specifying where, without recalculation, then for some reason getting a new basket object for the current user, which is not used anywhere. And check the coupon is working, the product in the cart matches the conditions