Laravel\Cashier\Billable::applyCoupon PHP Method

    public function applyCoupon($coupon, $subscription = 'default', $removeOthers = false)
    {
        $subscription = $this->subscription($subscription);
        if (!$subscription) {
            throw new InvalidArgumentException("Unable to apply coupon. Subscription does not exist.");
        }
        $subscription->applyCoupon($coupon, $removeOthers);
    }