If NiFi is configured to run securely, users will be able to request access to the DataFlow. If NiFi supports anonymous access, users will be given access accordingly and given an option to log in.
Clicking the 'login' link will open the log in page. If the user is logging in with their username/password they will be presented with a form to do so. If NiFi is not configured to support anonymous access and the user is logging in with their username/password, they will be immediately sent to the login form bypassing the canvas.