Unable to connect to the mycloud server for using time machine on my iMac

Hi there, I am absolutely new here. I have bought a WD my cloud home as a replacement for my time capsule. Setup was fairly easy, but I get no connection to time machine. As soon as I try to select the mycloud home as the backup volume, I am asked to connect as a registred user and to fill in a password. Unfortunately I do not know, which password is required here. The mycloud account password does not match. When I go via IP address, the answer is no key, no message - has anyone an idea how to handle that problem? I really appreciate any useful help.
Regards Axel