json-ld: Good way to model custom values

I'm trying to get a good json-ld that combines the schema.org/Product definition with some custom elements.

I'm coming from an xsd background and the extensibility in json-ld seems very difficult to achieve.

I started from the template markup for Products found at Google (https://developers.google.com/search/docs/guides/search-gallery) and tried to extend it (I would like to add something like mydomain:tags to it) but I'm not sure how to do this.

<script type="application/ld+json">
{
  "@context": ["http://schema.org/",
    {"mydomain": "http://mystuff.com/"}],
  "@type": "Product",
  "name": "Executive Anvil",
  "image": "http://www.example.com/anvil_executive.jpg",
  "description": "Sleeker than ACME's Classic Anvil, the Executive Anvil is perfect for the business traveler looking for something to drop from a height.",
  "mpn": "925872",
  "brand": {
    "@type": "Thing",
    "name": "ACME"
  },
  "aggregateRating": {
    "@type": "AggregateRating",
    "ratingValue": "4.4",
    "reviewCount": "89"
  },
  "offers": {
    "@type": "Offer",
    "priceCurrency": "USD",
    "price": "119.99",
    "priceValidUntil": "2020-11-05",
    "itemCondition": "http://schema.org/UsedCondition",
    "availability": "http://schema.org/InStock",
    "seller": {
      "@type": "Organization",
      "name": "Executive Objects"
    }
  },
  "mydomain:tags" : {}
}
</script>

Any clue on what I'm doing wrong here would be much appreciated. It's probably something silly...