In thist post I will show you how to configure your custom certificate on proxmox.
You can do this by copying the certificate and the key to the correct files and restart the pveproxy service.
cp fullchain.pem /etc/pve/local/pveproxy-ssl.pem cp privkey.pem /etc/pve/local/pveproxy-ssl.key systemctl restart pveproxy
From the Proxmox 6 you have an option to do this by the Web UI:
Note that the certificates key file, if provided, must NOT be password protected.
Proxmox Backup Server
cp fullchain.pem /etc/proxmox-backup/proxy.pem cp privkey.pem /etc/proxmox-backup/proxy.key systemctl restart proxmox-backup-proxy.service systemctl restart proxmox-backup.service