Hello,

I could suggest following these steps here from the official Plesk documentation on how to configure that with DigitalOcean:

https://docs.plesk.com/en-US/obsidian/administrator-guide/backing-up-and-restoration/configuring-remote-storage/securing-backups-in-remote-storage-with-a-password.59259/#digitalocean-spaces-storage

• Create a space. *Create a directory for storing backups inside the space. Note the path to this directory and the space URL. You will use them later to configure DigitalOcean Spaces storage.
• Create an access key and secret. Note them because you will use them later to configure DigitalOcean Spaces storage. Keep in mind that the secret is only displayed once, when it is created. There is no way to retrieve it later.
• Go to Tools & Settings > Backup Manager > Remote Storage Settings > Amazon S3 Backup.
• Under “Service provider”, select “DigitalOcean Spaces”.
• Paste the space URL, the path to the directory where the backups will be stored, the access key, and the secret into the corresponding fields, and then click OK.

Then once that is configured, setup the location where to store the backup files:

https://docs.plesk.com/en-US/obsidian/administrator-guide/backing-up-and-restoration/backing-up-the-entire-server.59257/

Hope that this helps!
Regards,
Bobby