Upload your files to the htdocs directory but don't specify the directory when requesting the content.
Use the following schema to form the URLs:
http://<storage name>.<hostname>/<file name>
Storage name and hostname can be found on the Storage tab -> Storages -> Details.
Example
Path to the uploaded file in the storage:
sftp://4505-test@ams.origin.gcdn.co:2200/export/home/4505-test/htdocs/picture.png
Storage name:
4505-test
Hostname:
ams.origin.gcdn.co
URL to request the file from the storage:
http://4505-test.ams.origin.gcdn.co/picture.png
SFTP storage works only via HTTP protocol.