How create a <domain> in a managed account that already exists?

I have create my account to manage my own domain and its working and done.

But Im now looking to how can I use the same account (same email if possible) to create a subdomain under (like Is it even possible?

When I try to create the domain it ask me to update the records but I cant do it of course…

---- edit solved ----

Ok turns out that the domain actually works normal, even it asking you to update the NS records it is working already. Just created the A record and its responding normal.

Sorry for the useless thread. Solved.