If you only want to point a subdomain to Uscreen because you want to use your www or root (naked) domains for other purposes you can create a CNAME record to point to Uscreen.
Navigate to the domain you want to setup under My Domains and click the three dot menu and Manage DNS. Under Records, you'll be either adding an 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
Host - whatever you want the subdomain to be i.e. tv or catalog
Points to - dns.uscreen.io
TTL - 1 Hour