Typed out an entire prompt on the homepage, hit enter, was redirected to sign-up & completed it. My few hundred word prompt wasn't saved/persisted. :'(
Half of the reason appears to be how the Oauth 2.0 standard is written - it purposefully forbids browser-specific items like the hash fragment in the URL. For passing state you have the "state" field but, despite the name, its original purpose was to prevent XSRF attacks, not store data.
Vendor libraries don't go to huge lengths to help here, as they settle on complying with the standard.