How to modify username in a Server Provisioning Module?

Hello.

I'm writing a server provisioning module ( http://developers.whmcs.com/provisioning-modules/ )

In running CreateAccount function, I need to modify somehow the default username created by WHMCS for the service (this because it's the server itself that, during account creation, create his own username).

That's to say:

- customer order the service

- WHMCS create a default, random username and a random password

- calling CreateAccount, the service is created on the server, and the server itself create username and password for the service

- to update the password, i can use the ChangePassword function

But... how to change username too?!?!

$username = 'admin';
$password = '123';
$params['model']->serviceProperties->save(['Username' => $username, 'Password' => $password]);

https://docs.whmcs.com/Service_Properties