# Customer Key

A customer key is a user-defined identifier for an asset. Each customer key is a unique value across your enterprise. The customer key isn’t a required field, and if you don’t provide a value, Marketing Cloud Engagement generates a unique GUID value for the customer key.

Add a customer key to the asset as a string with a 36-character limit, using the `customerKey` attribute.

## Sample Asset with Customer Key

```json
{
  "name": "NTO Welcome Series Email",
  "assetType": {
    "id": 207,
    "name": "templatebasedemail"
  },
  "customerKey": "090c69e7-59ac-4286-8733-f8f76example"
}
```

## Related Items

- [Asset Model](/docs/marketing/marketing-cloud/guide/asset-model.md)
- [Asset Model Example](/docs/marketing/marketing-cloud/guide/asset_model_examples.md)