They make it very hard to use your own "Box B", but I've set this up twice now (most recently last week). Get the username and password from CenturyLink (the tech that installs the service has this, or call them). Then, google search "century link vlan 201 wan tag". The trick is you need a router that has this functionality, most basic consumer ones don't.
Unfortunately, even if you follow all directions and it still doesn't work troubleshooting is a nightmare, very little or no help from their customer support.