How to hide username from login screen?

Username will be shown at login screen after creating user by useradd. But, there are those who would like username not to be shown at login screen, I want to make username disappeared at least. The command usermod enables this.

usermod -u 999 $UsernameNotToBeShownAtLoginScreen

I tried "gsettings set org.gnome.login-screen disable-user-list true" also, but there was no effect. This command seems to do something others. If that helps, "gsettings get org.gnome.login-screen disable-user-list" tells the current disable-user-list value.