Below an nmtui example with 10.0.0.1 being my router and home.dehakkelaar.nl the domain configure in the Pi-hole DHCP service:
You can configure the default domain name displayed in the DHCP service settings.
Or "home.arpa", "internal" or "lan".
EDIT: I prefer home.arpa as its intended for this purpose and that name cant be stolen:
$ whois home.arpa.
[..]
contact: technical
name: Internet Assigned Numbers Authority (IANA)
[..]
remarks: This domain is administered as part of the .ARPA zone
remarks: management, described at https://iana.org/domains/arpa