Addon domains form should pre-fill sub-domain with unique value

As a web hosting provider, our support team are regularly contacted by clients who encounter problems creating addon domains because "the subdomain already exists". We have to explain to users that when creating an addon domain in cPanel, the sub-domain box must be amended to contain a unique value that does not conflict with other sub-domains (despite the value being auto-filled by cPanel).

For example:

addondomain.com and addondomain.net will both suggest "addondomain.example.com" as the sub-domain value, so creating the second addon domain will cause the error "the subdomain addondomain.example.com already exists"

The cPanel form should be able to detect when the subdomain will conflict with an existing subdomain and auto-fill it with another value.