Custom DNS names?

I am looking to be able to add custom DNS names on my network. Example browsing to \\nas to get to my nas. In the "Client Devices" tab, I can see "local DNS Record" but it seems to want a FQDN (nas.domain.com). The same in settings > Routing > DNS. I can create a custom host but it wants FQDN.

Can I do what I want? Am I just dumb?