Your workspace ID (this appears as app _id in your code) is a unique code assigned to your app when you create it in Intercom. There are a few ways you can find your workspace ID.
The easiest way to find your workspace ID is to check the URL of any page you have open in Intercom. It's the code that comes after apps/ in the URL. So for this example app, if we check the URL we can see that the workspace ID is e8y21262.
Your workspace ID is also available on the top right of the page in the API keys section of your app settings. Lastly, your workspace ID is available for quick copy/pasting in your Setup Guide, in the right-hand-side column.