To access the Horizon dashboard you simply need to add '/wp-admin' to the end of the URL like the below example: www.example.com/wp-admin
This will then take you to the login screen, where you will enter your username and password. Then press 'Log In'.
Additionally, if you have forgotten your password you can click the 'Lost your password?' link, which will send you a temporary password once you follow the on-screen instructions.