If no password has been configured, you need to open the notebook server with its login token in the URL, or paste it above. This requirement will be lifted if you enable a password.
jupyter notebook listwill show you the URLs of running servers with their tokens, which you can copy and paste into your browser. For example:
Currently running servers: http://localhost:8888/?token=c8de56fa... :: /Users/you/notebooks
or you can paste just the token value into the password field on this page.
See the documentation on how to enable a password in place of token authentication, if you would like to avoid dealing with random tokens.
Cookies are required for authenticated access to notebooks.