First, you will need to buy a domain name from a DNS provider, such as Namecheap or Pair Domains. (Note: These are not endorsements, just examples.)
Next, go to your provider's settings. It should show a list of DNS records. Add a new record. Choose the CNAME type and set the value to user.tjhsst.edu.
Note: In the past, hardcoding the IP with an A or AAAA entry would also have worked. However, things have changed and this is no longer the case. Please use a CNAME.
Finally, click the "Configure Site" button on your site's page and enter the name of your custom domain in one of the "Custom domain" boxes.