actuel-inactuel/site/plugins/your-plugin/index.php

<?php

Kirby::plugin('getkirby/pluginkit', [
  'fields' => [
    'demo' => [
      'props' => [
        'value' => function ($value = null) {
          $margins = Yaml::decode($value);
          return [
            'top' => 0,
            'right' => 0,
            'bottom' => 0,
            'left' => 0,
            ...$margins
          ];
        }
      ]
    ]
  ],
]);