To add link to any page created with Pineapple Builder to a button or text link, open "Pages" tab and click more options for the page you wish to link to:
Copy of slug (URL) will be copied to your clipboard. In this case it will be
Now pick eg. button component you to which you want to add link and click "Edit"
This will open tab on the right with more options. In here you can paste from clipboard (ctr+v) link
Done, now when you preview this button on the page it will open your subpage.