class EntityResourcePostRouteSubscriber

class EntityResourcePostRouteSubscriber implements EventSubscriberInterface {
    
    /**
     * The REST resource config storage.
     *
     * @var \Drupal\Core\Entity\EntityStorageInterface
     */
    protected $resourceConfigStorage;
    
    /**
     * Constructs a new EntityResourcePostRouteSubscriber instance.
     *
     * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
     *   The entity type manager.
     */
    public function __construct(EntityTypeManagerInterface $entity_type_manager) {
        $this->resourceConfigStorage = $entity_type_manager->getStorage('rest_resource_config');
    }
    
    /**
     * Provides routes on route rebuild time.
     *
     * @param \Drupal\Core\Routing\RouteBuildEvent $event
     *   The route build event.
     */
    public function onDynamicRouteEvent(RouteBuildEvent $event) {
        $route_collection = $event->getRouteCollection();
        $resource_configs = $this->resourceConfigStorage
            ->loadMultiple();
        // Iterate over all REST resource config entities.
        foreach ($resource_configs as $resource_config) {
            // We only care about REST resource config entities for the
            // \Drupal\rest\Plugin\rest\resource\EntityResource plugin.
            $plugin_id = $resource_config->toArray()['plugin_id'];
            if (!str_starts_with($plugin_id, 'entity')) {
                continue;
            }
            $entity_type_id = substr($plugin_id, 7);
            $rest_post_route_name = "rest.entity.{$entity_type_id}.POST";
            if ($rest_post_route = $route_collection->get($rest_post_route_name)) {
                // Create a route for the 'create' link relation type for this entity
                // type that uses the same route definition as the REST 'POST' route
                // which use that entity type.
                // @see \Drupal\Core\Entity\EntityBase::toUrl()
                $entity_create_route_name = "entity.{$entity_type_id}.create";
                $route_collection->add($entity_create_route_name, $rest_post_route);
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function getSubscribedEvents() : array {
        // Priority -10, to run after \Drupal\rest\Routing\ResourceRoutes, which has
        // priority 0.
        $events[RoutingEvents::DYNAMIC][] = [
            'onDynamicRouteEvent',
            -10,
        ];
        return $events;
    }

}