Multiple subdomains on a single endpoint are currently not supported.
Subdomains on custom CDN endpoints are added by creating specific DNS records and Let’s Encrypt certificates that require the root domain to be on DigitalOcean for validation.
You’ll also need to add a CNAME record pointing from the subdomain to the default CDN endpoint.
You can still use a custom subdomain if you prefer to manage your DNS with another provider. However, you’ll need to follow our documentation here to “bring your own certificate”:

https://www.digitalocean.com/docs/spaces/how-to/customize-cdn-endpoint/#bring-your-own-certificate