If you haven’t done so already, see the following WD Knowledgebase Article that appears to answer your question. Note the part in the directions where one creates/enters an encryption password.
Encrypting a RAID Volume on a WD My Cloud Device
- The instructions in the article below are designed for encrypting a RAID volume on a supported WD My Cloud device. Please make sure to save and backup the encryption key and the original password. Without the encryption key or the original password, it is not possible to access the encrypted data that is stored on the WD My Cloud device.