Once your storage is configured on your Proxmox VE server :
On the left side select Datacenter ⇒ storage ⇒ cloud-pbs.com
Select Edit button
On the Edit box :
Select the Encryption ⇒Upload an existing client encryption key ⇒ Browse to your key file ⇒ OK
The next form reminds you to backup your key.
Save the key in your password manager : At Cloud-pbs, we think that this is the easiest solution. Select the Copy Key button, paste it in your password manager and voila.
Download the key to a USB key : This is a way to store offline your key, on a device. Copy the downloaded file to a usb hard disk drive, and place this drive into a safe box.
Print as paperkey : The more secure but painfull solution the day you'll want to use it to restore.
Done, your backups will be encrypted but do not forget : you will need the encryption key to restore your backups.