There's no way to do that yet, when we first released the app we promised that it wouldn't replace the website — basically, that the option to download from web would remain there. There's alternative ways to authenticate using the server-side API: https://itch.io/docs/api/serverside — you could ask folks for their e-mail, check that they've indeed purchased the game using that e-mail, and send them an e-mail with their Discord key!