This event occurs when either the contact clicks the opt-out link or when information has been passed about the change in status from opt-in to opt-out.
Go to AUTOMATION PROCESSES => WORKFLOW
From the list of the elements in Workflow choose Event => Contact has been opted out
IMPORTANT: You can’t trigger Action: send email to the contact. Instead, you can assign a tag to the contact or add a note.
IMPORTANT: Triggering an event via Manage contacts will not work.