An ownCloud storage is a specialized webdav storage, with optimizations for ownCloud-ownCloud communication. See the webdav documentation to learn how to configure an ownCloud external storage.

When filling in the URL field, use the path to the root of the ownCloud installation, rather than the path to the WebDAV endpoint. So, for a server at https://example.com/owncloud, use https://example.com/owncloud and not https://example.com/owncloud/remote.php/dav.