David,
If you navigate to https://sprintr.home.mendix.com/index.html you should see the following (see image below)
There should be a button with the text Create App available. This is not linked to any license model AFAIK.
Be sure to be on the genaral buzz of your company and not in a specific buzz for a project. If you still do not see the button to create a new app, please contact mendix support.
Before using Studio Pro you need to signup with your company email address.