What do I enter when asked for FQDN (fully qualified domain name), for a PC that's not in a domain but in a WORKGROUP instead?
Workgroup computers don't have an FQDN unless you specifically configure them to have one. They have only a NetBIOS (single label) name.
Also, don't confuse the Primary DNS suffix with the DNS suffix search list as assigned by DHCP because they're two different things.
For Windows PCs you can check with:
FQDN = Hostname + Primary DNS Suffix
So in this case: PCNAME.local