I just set this up for one of my Virtualmin-managed servers recently –

First, follow the path "Virtualmin > Backup and Restore > Cloud Storage Providers > Amazon S3" to set up your Spaces particulars.

• You should have generated your keys for use here from the DO dashboard. Input them here. (Note: You'll need to input the "Spaces access keys" from your DO API page, and not a "Personal Access Token". Additionally, you'll need a pair of keys – the "Access Key" and the "Secret Key". You may need to generate a fresh pair or regenerate an existing pair if you can't see your Secret key on this page.)
• In the field "S3-compatible server hostname" in Virtualmin, input your Spaces endpoint – i.e. [region].digitaloceanspaces.com (e.g. ams3.digitaloceanspaces.com)
• In the field "Upload chunk size in MB", a value between 20 - 200 should be optimal. I use 50.
• Save.

Now, go to "Backup and Restore > Backup Virtual Servers" to setup/run your backup.

• Under "Destination and format", select "Amazon S3 bucket. Your keys should have been pre-filled.
• For the "Bucket and path field", enter your "spacename" (i.e. the xyz in "xyz.[region].digitaloceanspaces.com"), followed by the path of any folder you might have created or wish to create – e.g. [spacename]/my-folder-1 (Do not include a trailing slash.)
• If you're creating a folder, you might want to select the "Create destination directory?" check box. (The other options are up to you, and you can find more information on your options for Virtualmin backups here – https://www.virtualmin.com/documentation/system/backup-and-restore )
• Click the "Backup Now" button, and you should be good to go!