First of all, you must have a Deezer account, and then you must enter the credentials in the Bang & Olufsen app* to log in to the account on your music system. If you do not have a Deezer account, you can sign up via the Deezer webpage:
Note that Deezer may not be available in all markets.
*How to add your Deezer credentials to the Bang & Olufsen app