The first thing you have to do is write a hub or two on the Hub Pages.
Reckon after having written, a few hubs, you can ask for ... and fill in, a Proforma, requesting a specific "sub-domain name" be allotted to you.
Once you have been allotted, your sub-domain address, which could read something like
xyz dot hubpages dot com
After that, all you have to do is Sign in on the Internet and Enter your Sub domain address ... your Hub Pages Account will Open automatically.
It has worked for me, my sub domain is :