Custom Code Workflow Action to read email subject line

Hello,

I’m looking for advice on how to trigger a Custom Code Workflow when an email is received so I can parse the subject line. I have 2 questions, what is the best trigger to use, and which API endpoint should I be able to call to get the email subject?
So far I see only 1 possible trigger and that is when a conversation object is created, but that event only give a conversation id and not an email id.

You can use the “conversation created” trigger to start the Custom Code Workflow. While it gives a conversation ID instead of an email ID. This allows you to parse the subject as needed in your workflow.

Which API endpoint can I use to get the subject of the conversation by it Id?