When creating DNS RRsets just enter a value in the Subname field. E.g.: Subname: test, domain: example.com. will yield the subdomain test.example.com..
There is a second more complicated way: You could set up a separate zone for the subdomain. E.g. zone 1: example.com. and zone 2: test.example.com.. These need to be connected with proper DS records in the example.com. zone. In most cases this is not necessary though.
HTH
fiwswe