When adding an Addon Domain in cPanel, it requires a Subdomain of your primary domain (the one you signed up with), and a Document Root (where it's website would be hosted). Because we do not use cPanel for web hosting, these are excess details that simply must exist to fulfill the needs of the control panel, and can be considered cosmetic and useless to you.
Previously, we had hidden these fields and generated a UDID on page load that would silently populate these fields. On rare occasions, however, the UDID generated by the cPanel function would not meet the requirements for the field, and would be rejected. This would require a page reload and another try to add the domain. At the time, this was deemed an acceptable limitation due to the frequent chargebacks that customers would issue upon visibility of the "subdomain" field (and they would openly state this as the reason). Until, however, the error strangely became more common on the Eagle server, leading to a chargeback as well. As chargebacks are costly to us, we have been trying to do our best to avoid them, especially where they occur simply because a feature is not beautiful enough in it's current iteration.
We have rolled back this page to it's original state, and it now appears with these Subdomain and Document Root fields. When you fill in the addon domain and hit Tab, the other fields populate themselves. On the rare occasion that the subdomain field is not unique and therefore fails (adding multiple of the same domain with different TLDs can cause this), simply fill any unique value there and be done with it.
We will continue to work toward a better and more seamless solution for adding domains without this annoyance, and hope that this is not too large of an inconvenience.