Server password - module or system?

[amon]

We are finishing configuring our system. A short question:

 

Is it a server module (whm, plesk or other module) which generates the server/hosting username (based on domain) or is it some other files/functions of your system? We would like to use custom usernames (based on our own criteria) and would write our own server module if this is where server usernames are generated.

 

If it is not the module, maybe you happen to know which WHMCS file is responsible for this?

 

Thanks!