There is no automatic way to do this, but you can try this:
- Go to your Facebook Page
- Open Messages tab
- Message people to see if they want to subscribe
- If they respond – they will get added to your ManyChat list.
- Tag people that replied "no" with the "unsubscribed" tag, and then use it later as a filter when you do a broadcast, so you only send messages to people who want your content. People can also subscribe to your bot by typing the system keywords, such as "Start" and "Subscribe" (they are not case-sensitive).