LukePOLO\LaraCart\LaraCart::addCoupon PHP Method

    public function addCoupon(CouponContract $coupon)
    {
        if (!$this->cart->multipleCoupons) {
            $this->cart->coupons = [];
        }
        $this->cart->coupons[$coupon->code] = $coupon;
        $this->update();
    }