Applying discount via plugin

-- HikaShop version -- : 2.5.0

I have a plugin which processes a coupon against certain rules. I have a requirement that when product A is added to the cart, and the coupon matches a rule, product B is added to the cart. Product B should be discounted by 100%.

Using the function onAfterCouponCheck I add product B when required:

$product_id = 'B';
$quantity = '10';
$cartClass = hikashop_get('class.cart');
$cartClass->update($product_id,$quantity);

Is there a way that I can apply a discount to product B when it is added to the cart so that it is a no cost addition to the cart.

Many thanks

Thank you for your response. I can achieve the discount by modifying the product price as you suggested. The unit price and total price for Product B are correct in the cart, however the subtotal is not correct. The Subtotal/Total pricing etc does not appear to be calculated using the modified prices.