Adding a user to an Windows EC2 instance as a local user

I'm creating a cloudformation stack for a user to launch a Windows EC2 instance from a Service Catalog.

A user will enter their username and password and will be presented with the Remote Desktop IP, which they should be able to login with the credentials provided.

This isn't working as I can only login with the Admin username/password.

What needs to be added to the UserData section to add the user to the Windows EC2 instance as a local user? I can't find anything online.

Thanks!