Add property to DTO during serialization in Symfony

In a Controller, during serialization, using the Symfony Serializer, I need to dynamically add a property portal to an object, depending on an outside $portal value, which is a query argument.

$content = $response->getContent();
# Switching the complete class, just to set a property
$type = match ( $portal ) {
    'portal_a' => FooBarContainsCustomPropA::class."[]",
    'portal_b' => FooBarContainsCustomPropB::class."[]",
};
$my_object = $this->serializer->deserialize(
    $response->getContent(),
    $type,
    'json',
    [ UnwrappingDenormalizer::UNWRAP_PATH => '[data][someObjectList]' ]
);

In this scenario, I am using custom objects, extending a main class, to set the property (hard-coded). I am searching for a better solution as this way, I am forced to add one additional class per portal/ case, which will get unmaintainable with possibly hundreds of user defined portals.

# This solution depends on someone manually adding classes
# when a user adds a portal:

class FooBar {
   # various properties
}

class FooBarContainsCustomPropA extends FooBar {
   public string $portal = 'Portal A';
}

class FooBarContainsCustomPropB extends FooBar {
   public string $portal = 'Portal B';
}

Current output + the desired result:

{
   "some": "prop",
   "another": "property",
+   "portal": "Portal B"
}