Officially, not possible. There are many prior discussions on WD removing the remote map capability when they rolled out the OS 3 firmware for the My Cloud units. Here is one such recent discussion from a few days ago:
Unofficially there are a number of ways to remote map a My Cloud depending on which single bay My Cloud firmware your My Cloud is running, and if one is using a VPN connection to that local network. In certain cases one would have to modify the My Cloud firmware via SSH to allow for remote access mapping.
If one has the earlier first generation single bay My Cloud running v4.x firmware then it is possible to install an unsupported app/module which allows for remote mapping.