By using the action Opt-in contact you can create a rule that will automatically change a contact’s status from opt-out to opt-in.
Since then, that contact will be able to receive messages from the system. This action comes in handy if you want to try to reach out to customers who never opted in. For example, you can send a reminder to people, who at some point submitted their contact details through a contact form, and still visit the site, but never confirmed their subscription (as per the double opt-in model). As long as you can verify if the opted-out people are still interested in your content, using this action is a fine way to enlarge your list of subscribers.
Go to AUTOMATION PROCESSES => WORKFLOW
From the list of the elements in Workflow choose Action => Opt in
After the selection, connect the action with an event by means of an arrow.