While you are developing and testing your application, you can use the free Essentials Plan of the ArcGIS Developer Subscription. Before deploying your app in production, you will need to ensure that your application meets one of the following requirements:
Esri requires that when you use an ArcGIS Online basemap, Esri data services, or Esri API technology in your app you must also include Esri attribution. There are specific requirements for attribution you may be required to address in your app depending on how your app is built and the data it uses.
More information on attribution requirements can be found on the Attribution in your app page.