Now you will see all sorts of posts out there that you cannot register xenserver via cli.
And you have to use XenCenter.
LIES!! ALL LIES!!!
First go here: http://deliver.citrix.com/go/citrix/xenserver_renewal
And register to get a license.
You will get a email. It will contain a license file.
Copy that. Paste it into a file in your xenserver.
xe host-license-add host-uuid=(UID of server) license-file=(file_name)
output: License applied.