$code = CouponCodeFactory::createOne(['code' => 'FOOBAR_123']);

$response = $client->request('POST', '/cart', [
    'json' => [
        ...
        'code' => getIriForResource($code->object()), // Instead of findIriBy(['code' => 'FOOBAR_123']) or manually generating IRI                
    ],
]);

self::assertResponseIsSuccessful();