I have this (pseudo) domain setup, and hope to do any configuration via cPanel and/or WHM.

myclient.com - this hosts a development site and there is no public DNS for it yet (by choice)
mycompany.com - this is my company's domain name and it's publicly accessible.
myclient.mycompany.com - this is the domain I want to use to preview the myclient website during development.

Edit for clarity: All 3 are on the same server and same IP.

So, is it possible to point that subdomain (myclient.mycompany.com) to the webroot of myclient.com using cPanel or WHM?

I've looked at addon domains, redirects and parked domains but each of them can only relate to own account, or cannot be used with another domain if it's managed by the same server.

Any idea? :/