You can include the information from custom fields within email templates using shortcodes. To learn more about shortcodes, check out our article on Shortcodes For Email Templates.
If have do not already have a custom field, you will need to add one in the Custom Fields section
A custom field's shortcode is based on what is entered in "Field Name (Internal)"
This name is for internal reference. It can be used for custom links, shortcodes, and the API. Be careful when changing the Field Name as it may affect existing integrations.