If you only want to point a subdomain to Uscreen because you want to use your www or naked domains for other purposes you can create a CNAME record to point to Uscreen.
Navigate to the domain you want to setup under Domain List and click Manage. Under Advanced DNS, you'll be either adding a CNAME record for your subdomain if it doesn't exist or editing your existing one.
Regardless you should have one record with the following:
Type - CNAME
Name - whatever you want the subdomain to be i.e. tv or catalog
Value - dns.uscreen.io
TTL - 1 Hour