This is an Inbox+ feature, which is currently in Open Beta. Please note that this feature is in active development and functionality or specific directions outlined in this FAQ may change.
To trigger an automation from a message in Inbox+:
Navigate to the Inbox section
Navigate to the message you want to use as a trigger for your Automation
Hover over the message and click the Lightning icon
Select the automation you want to trigger
Click Enroll now to enroll the user into the Automation
Note: You can only trigger manual Automations from a message in your Inbox. Automated or scheduled workflows cannot be triggered this way.