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/v26/common/custom_parameter.proto)

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