ember-simple-auth.js

import ENV from '../config/environment';
import Configuration from 'ember-simple-auth/configuration';
import setupSession from 'ember-simple-auth/initializers/setup-session';
import setupSessionService from 'ember-simple-auth/initializers/setup-session-service';

export default {
  name: 'ember-simple-auth',

  initialize(registry) {
    const config = ENV['ember-simple-auth'] || {};

    if(config.baseURL === undefined) {
      config.baseURL = (ENV.rootURL !== undefined ? ENV.rootURL : ENV.baseURL)
    }

    Configuration.load(config);

    setupSession(registry);
    setupSessionService(registry);
  }
};

With this ember-simple-auth-initializer one can set
ENV['ember-simple-auth'] = { baseURL: '' } in config/environment.js