add new domain

Hello
Since 2013 I'm using Zoho mail with my domain name myname.tk
Now I want to attach a new domain as myname.ml
But zoho said to me: Sorry! Free domains with extensions such as .tk, .cf, .ml etc., are not supported.

How can I use this new domain ?

second question:
I have the free plan, how much will it cost to add a domain?

Thank you for your services