defineProperty

controllers.application\.js

import Controller from '@ember/controller';
import { decorate } from '../services/my-service';

export default class ApplicationController extends Controller {
  @decorate('foo')
  appName;
}

services.my-service\.js

import Service from '@ember/service';
import { computed, defineProperty } from '@ember/object';
import { inject } from '@ember/service';

const SERVICE_KEY = '__MY_SERVICE__'

export default class MyService extends Service {
  hash = {
   foo: 'bar'
  };
}

export function decorate(key) {
  return function(proto, propName, descriptor) {
    if (!proto[SERVICE_KEY]) {
      defineProperty(proto, SERVICE_KEY, inject('my-service'));
    }
    
    return computed(`${SERVICE_KEY}.hash.${key}`, function() {
      return this[SERVICE_KEY].hash[key];
    })(proto, propName, descriptor);
  }
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}