How do you get the

oauth2_login_challenge

value in the response body of the call to create a login flow?

If you run hydra by self hosting, you need to add login , logout, error, concent pages urls in hydra.yml or hydra config. Hydra will automatically add the login challenge. When the oauth client sends request to hydra auth url , hydra will automatically redirect to login url with login challenge. Once after your identity provider checks the identity you can decide to accept the login challenge or reject login challenge.