Unable to add subdomain email account under the main email account

Unable to add subdomain email account under the main email account

Hi,

Is it possible to add a subdomain email account, like sky@agent.rebrightrental.com from the Admin Console of admin@rebrightrental.com in Zoho Mail? Or is that not possible? I want to create at least 3, like oak@agent.rebrightrental.com, bliss@agent.rebrightrental.com, etc. 

Where am I registering or adding the subdomain? When I go to "add" the subdomain in my Admin Console, because I'm on the free plan, it says I've reached the maximum amount of domains to add. But I don't want to add another main domain, I just want to add a subdomain. 

Thank you.