controller.php

<?php

namespace App\Http\Controllers;

use App\User;
use Illuminate\Http\Request;

class PlansController extends Controller
{
  
  public function save(Request $request, string $plan)
  {
      /** @var User $user */
      $user = $request->user();
      $token = $request->get('paymentMethodId');
      $couponCode = $request->get('couponCode');

      // Build the subscription object.
      $subscriptionBuilder = $user->newSubscription('default', $plan);
      if ($couponCode) {
          $subscriptionBuilder->withCoupon($couponCode);
      }

      // Create the subscription.
      $subscriptionBuilder->create($token);

      return redirect(route('home'));
  }
}