This seems related to https://support.freshdesk.com/support/discussions/topics/304230 (use custom fields in email response). Rather than dribble all these requests (which all amount to (treat customer fields like any other field..), I've created a new request to simply have them act - in all ways - like canned fields. Please add your votes.
about 1 year ago
The custom fields can be added as placeholders in canned responses/email notifications using the Insert placeholder option.