Do not know the exact method, but can be done this way.
Instead of adding it as a subdomain, add it as a domain.
I mean if the domain is xyz.com, and subdomain is 123.xyz.com, instead of creating a subdomain in xyz.com, go to the reseller account and click on Add new Domain and add 123.xyz.com and the rest of the things like you are adding a new domain.
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