This process has been greatly simplified in recent weeks.
You can now download an app which you can install into a universal forwarder from the sandbox instance itself.
After logging into your instance, click on the "Universal Forwarder" app from the launcher page.
From the subsequent page you can download the app and follow the instructions to install it into a universal forwarder.
Also,
The answers given above were valid at the time of writing but recently we secured all data inputs with a unique SSL certificate and key for each instance so you cannot just manually add the config files and make it work anymore. You have to download the universal forwarder app which has the required credentials embedded.
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