Open a terminal of your PVE host (be it via SSH or via the pve webui) and check which Debian release you are using. PVE 8 is based off of Debian 12 codenamed "Bookworm".

deb http://ftp.si.debian.org/debian bookworm main contrib

deb http://ftp.si.debian.org/debian bookworm-updates main contrib

# security updates
deb http://security.debian.org bookworm-security main contrib

deb http://download.proxmox.com/debian/pve bookworm pve-no-subscription

Then add the HP signing key. Other tutorials provide two more keys, but those are - to my understanding - not required for this repository, so we'll only be adding this one.

curl -fsSL https://downloads.linux.hpe.com/SDR/hpePublicKey2048_key1.pub | gpg --dearmor > /etc/apt/keyrings/hpePublicKey2048_key1.gpg
chmod 0644 /etc/apt/keyrings/hpePublicKey2048_key1.gpg

And then add the repository to your APT sources (make sure you use correct OS release codename - in this case "bookworm"), update and install ssacli package

cat <<EOT > /etc/apt/sources.list.d/hp-mcp.list
deb [signed-by=/etc/apt/keyrings/hpePublicKey2048_key1.gpg] http://downloads.linux.hpe.com/SDR/repo/mcp bookworm/current non-free
EOT

apt update && apt install -y ssacli

You will get the following warning after updating the local APT index. You can safely ignore it as HPE (at least as of 31th of Oct 2024) have not migrated to the new format yet

N: Repository 'Debian bookworm' changed its 'non-free component' value from 'non-free' to 'non-free non-free-firmware'
N: More information about this can be found online in the Release notes at: https://www.debian.org/releases/bookworm/amd64/release-notes/ch-information.html#non-free-split

After successful installation you will be able to use the ssacli utility and (for example) list detected controllers in your system

root@ethos:~# ssacli ctrl all show

Smart Array P840 in Slot 3                (sn: PDNNF0ARH19A7M)

root@ethos:~#