Add a dynamic property to API Platform documentation

Documentation.php

<?php
declare(strict_types=1);

namespace App\Api;

use ArrayObject;
use Symfony\Component\Serializer\Normalizer\NormalizerInterface;

final class Documentation implements NormalizerInterface
{
    /**
     * @var NormalizerInterface
     */
    private $decorated;

    public function __construct(NormalizerInterface $decorated)
    {
        $this->decorated = $decorated;
    }

    public function supportsNormalization($data, string $format = null): bool
    {
        return $this->decorated->supportsNormalization($data, $format);
    }

    public function normalize($object, string $format = null, array $context = [])
    {
        /** @var ArrayObject[] $docs */
        $docs = $this->decorated->normalize($object, $format, $context);

        // Use your object shorthand instead of MyObject and the dynamic property name instead of myDynamicProperty
        $docs['definitions']['MyObject']['properties']['myDynamicProperty'] = [
            'type' => 'string',
            'example' => 'Example code here',
        ];

        return $docs;
    }
}

services.yaml

services:
    _defaults:
        autowire: true
        autoconfigure: true

    App\Api\Documentation:
        decorates: 'api_platform.swagger.normalizer.api_gateway'