You need to request access to the Sandbox first. In the dashboard, there is a button that automatically creates a support ticket for you. After that, the Stack Name, user, password and other information will appear on your dashboard.

The Sandbox request will take some days to get processed.

This actually can be found in a few places. My go to way to find this is https://q2developer.com/dashboard there is a section in the Developer Information card that says "Stack Name" the stack name should also be provided in the welcome email.