Hi Alex,
I tried the following URL and it worked for me:
Code:
https://10.1.1.1:2083/xml-api/cpanel?cpanel_xmlapi_version=2&cpanel_xmlapi_module=AddonDomain&cpanel_xmlapi_func=addaddondomain&newdomain=alex.net&subdomain=alex.dave.com&dir=alex
Are you authenticating your call as the user? (in my case it's "dave", how owns "dave.com"; in your case it would be the user who owns "alex.com"). If you authenticating as 'root', then you should use port 2087 and also provide the URL parameter "cpanel_xmlapi_user=$cpuser".
Regards,
-DavidN