So I have a website which uses mysqli to login. I installed indico on indico.mysite.eu and I want to be able to login only once for both mysite.eu and indico.mysite.eu. Thankyou.
Sharing logins without even clicking the login button etc: No, not really possible.
Using some existing user database for logging in: Sure, you just need to implement a flask-multipass auth provider for it.