Update a product custom field

const options = {
  method: 'PATCH',
  headers: {
    'x-spree-api-key': '<api-key>',
    Authorization: 'Bearer <token>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({value: '<unknown>'})
};

fetch('http://{defaultHost}/api/v3/admin/products/{product_id}/custom_fields/{id}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

{
  "id": "cf_UkLWZg9DAJ",
  "label": "Title",
  "type": "Spree::Metafields::ShortText",
  "field_type": "short_text",
  "key": "custom.title",
  "value": "cotton",
  "created_at": "2026-05-21T18:11:34.022Z",
  "updated_at": "2026-05-21T18:11:34.306Z",
  "storefront_visible": true,
  "custom_field_definition_id": "cfdef_UkLWZg9DAJ"
}