Officially you cannot remote map a My Cloud drive. WD removed that capability last year. Officially the only methods of remote access are; WD MY Cloud Desktop, WD My Cloud mobile apps for iOS/Android, MyCloud.com web portal, WD Sync, and the insecure FTP access.
If you do a forum search you’ll find one or two discussions on trying to activate SFTP that if successful one could use the SFTP feature of Free File Sync to access the My Cloud remotely.
Unofficially, and absolutely not supported by WD, if one has the older v4.x My Cloud they could install the CloudDAVMod app that will allow one to remote map their My Cloud.
https://community.wd.com/t/app-clouddavmod-various-versions-for-firmware-v4-09-2016/161543
