Currently there's no mechanism for an application to automatically login a user that
is already logged in to the realm.
I've added a proposal to
https://github.com/stianst/keycloak/tree/auto-sso. It's a
simple approach where all it does is to add an optional 'noforms' query parameter
to 'auth/request/login'. If noforms is specified a code is returned only if the
user is already logged in to the realm + grants are already given (as grants are not saved
currently that will never be the case). Otherwise it will return error=access_denied.