How can I enable two-click unsubscribe link or add a button to click on the unsubscribe link?

By default clicking on the unsubscribe link will automatically unsubscribe the recipient from all future campaigns. (You can add or remove recipients manually from the "Unsubscribers" menu.)

If you would like the unsubscribe link to ask the user to confirm they want to unsubscribe then enable two-click unsubscribe from your account settings page.