SaveIncludedCustomerUser.php

<?php

namespace Oro\Bundle\CustomerBundle\Api\Processor\Create;

use Oro\Bundle\CustomerBundle\Entity\CustomerUser;
use Oro\Bundle\CustomerBundle\Entity\CustomerUserManager;
use Oro\Component\ChainProcessor\ContextInterface;
use Oro\Component\ChainProcessor\ProcessorInterface;
use Oro\Bundle\ApiBundle\Processor\Create\CreateContext;

class SaveIncludedCustomerUser implements ProcessorInterface
{
    /**
     * @var CustomerUserManager
     */
    protected $userManager;

    /**
     * @param CustomerUserManager $userManager
     */
    public function __construct(CustomerUserManager $userManager)
    {
        $this->userManager = $userManager;
    }

    /**
     * {@inheritdoc}
     */
    public function process(ContextInterface $context)
    {
        /** @var CreateContext $context */

        $includedEntities = $context->getIncludedEntities();
        if (null === $includedEntities) {
            // the Context does not have included entities
            return;
        }

        foreach ($includedEntities as $entity) {
            if ($entity instanceof CustomerUser) {
                $this->userManager->updateUser($entity);
            }
        }
    }
}

services_api.yml

 # file Oro/Bundle/CustomerBundle/Resources/config/services_api.yml
 oro_customer_user.api.create.save_included_entity:
        class: Oro\Bundle\CustomerBundle\Api\Processor\Create\SaveIncludedCustomerUser
        arguments:
            - '@oro_customer_user.manager'
        tags:
            - { name: oro.api.processor, action: create, group: save_data, priority: 1}