> [!CAUTION]
> **Caution:** This documentation is for Google Ads API **v23** . The latest active version is [**v26**](https://developers.google.com/google-ads/api/reference/rpc/latest/CustomParameter).

A mapping that can be used by custom parameter tags in a `tracking_url_template`, `final_urls`, or `mobile_final_urls`.

[Proto definition ![GitHub](https://developers.google.com/site-assets/logo-github.svg)](https://github.com/googleapis/googleapis/blob/master/google/ads/googleads/v23/common/custom_parameter.proto)

| Fields ||
|---|---|
| ## `key` | `string` The key matching the parameter tag name. |
| ## `value` | `string` The value to be substituted. |