How do I set up an mailadmin user for existing domain?

How do I set up a webadmin user on a domain that already exists on my account. I have a client that whose domain I set up on my account that wants mailadmin access to his domain. I have tried through …/reseller/create-user/ but I received an error stating “domain already exists”.

First, add the cname record for mailadmin to DNS for the domain, then on that user’s account re-issue the SSL certificate for the domain and it’s subdomains.

It may be helpful to revisit the tutorial on this:

If the domain already exists on one DirectAdmin account on the server, it would need to be deleted from that account and added back on another account.