It's a blend of the two threads here:

https://communities.vmware.com/t5/Skyline-Health-Diagnostics-SHD/Name-Resolution-on-SHD-appliance/m-...

https://communities.vmware.com/t5/Skyline-Health-Diagnostics-SHD/How-do-I-change-the-DNS-settings-on...

Just add that Domain=xy.local to the /etc/systemd/network/*.network file and restart the systemd-resolved & systemd-networkd with the commands 

systemctl restart systemd-networkd

systemctl restart systemd-resolved

Good Luck!