Target One Particular Webform?

Problem/Motivation

Is there a way to target a particular webform with the key parameter? I'm trying to use Mailchimp Transactional to send a webform autoreply for just one webform. For key, I tried the id of the webform, and also the id of the autoreply email itself, but neither worked - no mail was sent. It works fine if I just leave the key field blank when setting up the Module-specific Configuration for webforms, but then that uses Mailchimp Transactional for all webform emails.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes