title: applying promotion code instead of coupon in laravel/cashier
body: <p>i am trying to use Promotion code instead of coupon code in laravel cashier/stripe official package but i couldn't find any help in there on how to apply promotion code instead of coupon on subscription as i know how to apply coupon but i want to apply promotion code, how can i do it.
  below is my code for applying coupon when creating new subscription but i don't know how can i achieve it using promotion code:</p>
  <pre><code>$subscription = $user-&gt;newSubscription($plan-&gt;name, $selectedPlan);
  
              // Add the trial days if any
              if ($plan-&gt;trial_days) {
                  $subscription = $subscription-&gt;trialDays($plan-&gt;trial_days);
              }
  
              // Add the coupon id if any
              if ($request-&gt;input('coupon_id')) {
                  $subscription = $subscription-&gt;withCoupon($request-&gt;input('coupon_id'));
              }
  
              $subscription-&gt;create($paymentMethod-&gt;id, [
                  'email' =&gt; $user-&gt;email
              ]);
  </code></pre>