According to /usr/share/doc/qubes-core-admin-addon-whonix/README,md, the template has to be given the feature whonix-ws = 1:
qvm-features whonix-ws-14 whonix-ws 1
Afterwards, new VMs based whonix-ws-14 do get the anon-vm tag and have dispvm set to whonix-ws-14-dvm.
In addition, VMs created using qvm-create get netvm set to sys-whonix. Those created from the GUI get whatever netvm was set in the GUI.