To register for the CloudSandbox application your need to create an App registration in Azure. You could use the following instructions to create one, or go for more details to the official Microsoft : website for creating an App registration.
To create an App registation in Azure, follow these steps.
1) Login to portal.azure.com.
2) Select Azure Active directory -> App registration (preview) -> New Registration.
3) A new window will pop up, where you need to fill in a chosen name and click on register. Make sure you select "Accounts in any organizational directory"
4) After registering, the App registration will automatically open. Note down the Application ID and the Directory tenant ID.
5) Lastly, you need to create a secret for this App registration. Click on Certificates & Secrets -> New client secret ID -> select 2 years -> Add. Note down the secret Key Value you see.
6) With these three settings: Application ID, Tenant ID and Client Secret you can fill in the registration form for the free version of CloudSandboxes.