It looks like this was answered, but I am putting here some more specific directions and an example. I think this will clear up any confusion that anyone might have, and at least its our here if anyone else is ever looking to do this and does not know how:
Login to domain registrar (the place where you got - whatevernameyouwant.com) , then manage DNS, then add A record to point to your servers IP address. Save/Apply. Here is an example using godaddy and adding an A record.
No quote
You will need to wait a bit (it should tell you how long) for the addition of the records to propagate out, probably about an hour maybe less. Then login to Xtream UI and go to Servers >> Manage Servers >> Then Choose EDIT Server. Put your domain name (whatevernameyouwant.com) into the Domain Name Field. Choose Next, then Edit. And it will save your settings changes. You don't have to but I would recommend restarting the server, or atleast the xtreamui service. Once that's done, drop a list with one of your lines to validate that the changes worked, and you should see your domain name populating and not your IP address anymore in the lines list.