How to obtain package object containing promotional offers?

I’m implementing 2 promotional offers, and have been following the official documentation. I tried to add the source code in the docs to obtain the promotional offers, like this:

if let discount = package.storeProduct.discounts.first {
    Purchases.shared.getPromotionalOffer(forProductDiscount: discount, product: package.storeProduct) { (promoOffer, error) in
        if let promoOffer = promoOffer {
            // Promotional Offer validated, show terms of your offer to your customers
        } else {
            // Promotional Offer was not validated, default to normal package terms
        }
    } 
}

but I don’t know how to obtain ‘package’, which appears in the first line of the above code.