Sorry for answering an old post, but this link is owrking for me:
https://web.archive.org/web/20211203064022/https://nixvsevil.com/posts/wireguard-in-proxmox-lxc/
I figured it out:
Add to /etc/apt/sources.list (or create a new file within /etc/apt/sources.list.d/) the following:
deb http://download.proxmox.com/debian stretch pve-no-subscription
Then make an apt update && apt upgrade -y