You can get it using below request (access token to be passed for Authorization header in Postman).
Get template-id from the URL (Administrators > Marketing Plans > Transactional Messages > Transactional Messages > Select the template )
https://<instance-name>.campaign.adobe.com/rest/head/delivery/<tempate-id>/emailPreview/?variant=default
There is also an option (under Content Source) to get the template from AEM where you can parse and change i t programmatically as needed.