[myapp].onhercules.app.
How do I publish my app to a free [myapp].onhercules.app domain?
Click Publish in the top right corner of the App Builder, then enter a custom subdomain name.

Can I change my Hercules domain?
Yes. Click publish, then click the pencil icon next to your subdomain. It will unpublish your app, then allow you to change your domain, then republish.