That's what I meant, I had to use virsh to set that up because it's not possible to do in the GUI.
virt-manager v4.0.0 (Ubuntu 22.04 amd64)
"QEMU/KVM" > "Details" (via double-click or context menu) > "Virtual Networks" tab > "+" (Add) button > "Create Virtual Network"/"Details" tab > Expand "IPv6 configuration" > check "Enable IPv6" > enter "Network" prefix and/or check "Enable DHCPv6" > enter "Start" and "End" addresses