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# Insert merge fields into callout templates

> Describes how to use merge fields to include related object data in the request URL, request body, HMAC authentication data, or custom headers of a callout template.

Describes how to use merge fields to include related object data in the request URL, request body, HMAC authentication data, or custom headers of a callout template.

When creating a callout template, on the Create Callout Template page, specify the **Field Type** and **Field** fields in the **Available Merge Fields** section. Zuora will auto-generate and display the corresponding merge field tag in the **Merge Field Tag** field. For example, `{{DataSource.Account.Id}}`.

Copy the merge field tag in the **Merge Field Tag** field.

Paste the merge field tag into any of the following places as needed:

* The **Request URL** field
* The **Request Body**, which must be a JSON-formatted object enclosed with curly braces ( `{}` )
* The **Data** or **Header Value** field of HMAC authentication
* The **Header Name** or **Header Value** field of custom callout headers