If you click Run (and it deploys to the sandbox cloud) then you should simply be able to click on View App. and it'll go straight to your app in the sandbox environment
Otherwise you can also go to your apps in the Sprinter environment (just click on apps in the main menu above this forum) navigate to your app. And select View app on the top right corner. That should also navigate you to your app in the sandbox environment.
If you want them to test against your localhost, you will need to use something like ngrok, which provides a public tunnel to your local machine. Or you could deploy to the Sandbox and then you would have a public URL to send your client.
Hope that helps,