Once you attach to the WiFi, it should fling you to a page that will tell you what to do. But essentially the summary is that once attached to the WiFi you need to open a web browser (Safari, Chrome, Firefox, there are others) and go to the address shop.ba.com

Quite why the onboard WiFi captive portal / initial web page can't immediately direct you to shop.ba.com I don't know, but that's what you need to know.