MyDataSource.php

<?php
namespace Vendor\Site\DataSource;

use Neos\Flow\Annotations as Flow;
use Neos\Flow\Persistence\PersistenceManagerInterface;
use Neos\Neos\Domain\Service\UserService;
use Neos\Neos\Service\DataSource\AbstractDataSource;
use Neos\ContentRepository\Domain\Model\NodeInterface;

class MyDataSource extends AbstractDataSource
{

    /**
     * @var string
     */
    static protected $identifier = 'vendor-site-mydata';

    /**
     * @Flow\Inject
     * @var \Neos\Flow\Http\Client\Browser
     */
    protected $browser;

    /**
     * @param NodeInterface $node The node that is currently edited (optional)
     * @param array $arguments Additional arguments (key / value)
     * @return array
     */
    public function getData(NodeInterface $node = null, array $arguments)
    {
        $response = $this->browser->request('https://your-symfony-service');
        return json_decode($response->getBody()->getContents(), true);
    }
}