Requests a URL which authenticates an in-game browser for store check-out, and then redirects to the specified URL. As long as the in-game browser accepts and handles session cookies, Steam microtransaction checkout pages will automatically recognize the user instead of presenting a login page. NOTE: The URL has a very short lifetime to prevent history-snooping attacks, so you should only call this API when you are about to launch the browser, or else immediately navigate to the result URL using a hidden browser window. NOTE: The resulting authorization cookie has an expiration time of one day, so it would be a good idea to request and visit a new auth URL every 12 hours.