It should be noted that FTP is NOT a secure protocol. Your use name and password are sent unencrypted and in the clear. If you want to use FTP you have to enable port forwarding in your router to pass traffic on the FTP port (21) to the My Cloud. See your routers documentation on how to enable port forwarding. The user would use their My Cloud User name and password to log into the share’s they have permission to access.
It is possible to configure the My Cloud to use SFTP which is a more secure access but it typically involves some editing of My Cloud firmware files via SSH to enable User access via SFTP. Instead of port forwarding port 21 for FTP, one port forwards port 22 for SFTP. Just note that there are additional security issues when one enables SFTP access to their My Cloud. The user would have full access to the My Cloud drive and could potentially mess around with the My Cloud OS files.
https://community.wd.com/t/sftp-setup/95350/4
https://community.wd.com/t/how-to-setup-sftp-on-my-cloud/152297
