MediaWiki:Citizen.js

$(document).ready(function() {
    setTimeout(function() {
        if (mw.user.isAnon()) {
            var elements = $('.cs-header > .oo-ui-widget-disabled');
            elements.each(function() {
                var $element = $(this);
                var loginMessage = '<p style="cs-custom-login-prompt">Please <a href="/wiki/Special:UserLogin">login</a> or <a href="/wiki/Special:CreateAccount">sign up</a> to continue.</p>';
                $element.after(loginMessage);
            });
        }
    }, 250);
});