curl --request PUT \
  --url https://api.bspk.com/api/platform/v1/shoppers/{shopper_ref}/metafields \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "metafields": [
    {
      "namespace": "custom",
      "key": "loyalty_tier",
      "value": "gold",
      "value_type": "single_line_text_field",
      "is_list": false
    }
  ]
}
'

[
  {
    "id": 123,
    "namespace": "<string>",
    "key": "<string>",
    "value": "<string>",
    "value_type": "<string>",
    "is_list": true,
    "created_at": "<string>",
    "updated_at": "<string>",
    "json_value": null
  }
]