Create a product custom field

import { createAdminClient } from '@spree/admin-sdk'

const client = createAdminClient({
  baseUrl: 'https://your-store.com',
  secretKey: 'sk_xxx',
})

const customField = await client.products.customFields.create('prod_UkLWZg9DAJ', {
  custom_field_definition_id: 'cfdef_AbC123XyZ',
  value: 'wool',
})

spree api post /products/{product_id}/custom_fields -d '{"custom_field_definition_id":"cfdef_AbC123XyZ","value":"wool"}'

{
  "id": "cf_gbHJdmfrXB",
  "label": "Description",
  "type": "Spree::Metafields::LongText",
  "field_type": "long_text",
  "key": "custom.description",
  "value": "A longer description",
  "created_at": "2026-07-26T14:25:34.628Z",
  "updated_at": "2026-07-26T14:25:34.628Z",
  "storefront_visible": true,
  "custom_field_definition_id": "cfdef_gbHJdmfrXB"
}

{
  "error": {
    "code": "validation_error",
    "message": "Metafield definition has already been taken",
    "details": {
      "metafield_definition_id": [
        "has already been taken"
      ]
    }
  }
}