'Click here for unsubscribing from these email campaigns'
'Opt-out from our mailing list from here' etc.
Open link option and paste [UNSUBSCRIBE_URL] in URL and change the protocol as <other> follows.
It will be now hyperlinked with Unsubscribe URL.
Kindly Note: Any Custom tag such as [FNAME] or [UNSUBSCRIBE_URL], will only work in actual campaigns, not in test/preview email.