Outlook rest api oauth2 php HTTP error 400 during CURL call
So you're saying you get an error in the sandbox? If you're getting the error that your mailbox is not enabled for the REST API, then that's the cause of the issue. Outlook.com accounts (Hotmail.com included) are being enabled in waves, and yours hasn't been enabled yet. At this time there's no way to enable them on demand, so for that mailbox you would just have to wait.
If you want to test right away, you can create a new Outlook.com account in North America, and it should be enabled for the APIs right away.