New CPU models can be defined here, node or cluster wide:
/etc/pve/virtual-guest/cpu-models.conf

Add this definition to the /etc/pve/virtual-guest/cpu-models.conf file:

cpu-model: x86-64-v2-AES-AVX
    flags +avx;+avx2;+xsave;+aes;+popcnt;+ssse3;+sse4_1;+sse4_2
    phys-bits host
    hidden 0
    hv-vendor-id proxmox
    reported-model kvm64

It will then be immediately available in the GUI as a Custom (QEMU) CPU:


Enjoy!

Reference:
https://pve.proxmox.com/wiki/Manual:_cpu-models.conf