So, providing some insight: usually when people mention FQDN, they refer to something like:
1. "prod.company.com"
2. however, it can be also something like "hostname.internal.company.com".
The second option is used as FQDN in the internal network, while the first one as FQDN for public networks.
The reality is that in the FQDN you can put whatever name will be resolved by the computer that will try accessing the ThingWorx instance.
For example, if you use as FQDN an internal resolvable name, and you only plan to access the instance from internal company network, then that will work perfect (but of course, that FQDN will not be resolvable from public internet, unless you use a VPN)
However, for public access, you will most probably use "prod.company.com". In this situation, you need to choose a subdomain name (the "prod" part).
Overall I you should raise an IT ticket in your company to ask them these details. These are usually well known by the IT teams.