Save as /etc/apt/sources.list.d/proxmox.sources (channel stable).

APT repository (DEB822)

Types: deb
URIs: https://repos.onesystems.ch/deb/proxmox/stable
Suites: stable
Components: main
Signed-By: /usr/share/keyrings/proxmox.gpg

GPG key (run once before apt update)

curl -fsSL https://repos.onesystems.ch/api/projects/proxmox/gpg-key \
  | gpg --dearmor | sudo tee /usr/share/keyrings/proxmox.gpg > /dev/null