Conditional order discount if a pseudo-product / option is selected.

When a customer checks an option on the product page, a free product (0€) is added to the cart

-> I want to automatically apply a 15€ discount on the order ONLY if this free product is present

-> Important: The discount must automatically disappear if the customer removes the free product from the cart

the added product is actually an option of the main product, it does not have to be materialized as a product. It is a box created with the Globo application that is checked. The addition of a free product was simply done to help the 15 euro discount to be applied in the cart.