adding subdomain of another site as addon to my example.epizy site

Hi there,
I need little support about adding a new subdomain of my other site to my example.epizy.com, I am trying to add it as addon URL because from now I want to use my cms.example.com as primary URL on my example.epizy.com site.

I’ve already pointed my servers to ns1.epizy.com and ns2.epizy.com. When I try to add my cms.example.com it says domain not registered.
Please guide me.
Is this going to change my site’s address permanently into cms.example.com?

In this example, is “example.com” a domain name you own and “cms.example.com” a subdomain of that domain you own?

If so, you will want to add “example.com” as an addon domain to your account. After you’ve set that up, you can add “cms.example.com” to your account as a Subdomain.