index.php

<?php
use DI\Container;
use DI\ContainerBuilder;
use Slim\App;
use Tuupola\JwtAuthentication;

$containerBuilder = new ContainerBuilder();
$containerBuilder->addDefinitions([
  'JwtAuthenticationSettings' => [
    'secret' => getenv('jwt_secret'),
    'path' => ['/admin/api'],
    'ignore' => ['/admin/api/login'],
    'error' => function (Response $response, $arguments) {
        return $response->withJson([
            'message' => $arguments['message'],
        ], 401);
    }
  ],
  JwtAuthentication::class => DI\create()->constructor(DI\get('JwtAuthenticationSettings'))
]);

$container = $containerBuilder->build();

$app = new App($container);
$app->group('whatever', function () {
  ...
})->add(JwtAuthentication::class);