Managed Instance Restart
You don't restart Managed Instance. The beauty of PaaS is that the cloud service provider ensures that you get a certain uptime SLA-- 99.99% in this case. 

The service is abstracted away. Microsoft will ensure that the service is "always" up and running. They handle patching and maintenance on the OS and the instance (and necessary restarts that go along with those things).

If you need to stop, scale, or make other changes to the instance, you'd do that from the Azure portal (either the web GUI or PowerShell).