You can call, and the agent can pick a seat for you, including extra legroom seats.

If you don't want to call unless absolutely necessary, you give the mobile app a try: sometimes the app allows you to pick a seat when the website doesn't. However I have lately noticed that it shows up on the website as 'unpaid regular seat' and threatens to boot me out if I don't "contact my booking source to finalize payment"... I never took any action and haven't been booted, but YMMV.