Unable to add Shared Mailbox to Zoho Mail "Premium Feature"

Unable to add Shared Mailbox to Zoho Mail "Premium Feature"

In Zoho Mail > Settings > Shared Mailbox I am not able to add Shared Inbox as it is marked as "Premium Feature".

How does it cost for a Zoho One subscriber? It says it's available for paid plans, which Zoho One is, how do I activate it?