So far it looks like you need to create the class in Scheduling
When done with that you need to go to ‘scheduling page link’ page
Select that class in the dropdown menu
Then click on the ‘direct link’ tab below the menu
Copy that direct link to that class
Then get out of scheduling, click ‘pages, classes and then the class you’ve created for this class (this page you are currently on)
Select the button and in the edit overly paste the copied link into the button
Make sure that link opens a new window, which is not the cleanest experience
There is a ‘workshop’ feature that would make this a bit smooth/less steps, but it is an extra yearly cost to have the commerce tools. (https://support.squarespace.com/hc/en-us/articles/115014778187-Selling-workshops) This commerce tool can also be used if you ever want to start selling physical goods.