Locate Requires Username, and use the toggle to enable or disable requiring a username.
To see how this will affect the login screen, select the Try Connection view. Notice that once Requires Username is enabled, new users must enter a username and their email address to sign up.
Users can then login with either their username or their email address. Users who registered while Requires Username was disabled will not have a Username field stored and will have to log in with their email address.
The default allowed length for usernames is between 1 and 15 characters. However, using the or via the v2, you can modify the length minimum and maximum (up to a maximum length of 128 characters).