I didn't even know this existed till I was way deep into configuring HA, but thats a good way to learn. Still, this helped me understand things conceptually and troubleshoot a resource problem.
In essence, you got to
And login with username "hacluster" and the password you authenticated the cluster with.
NOTE: Its always HTTPS, even if you don't have SSL Correctly setup, it will just say the usual warning etc etc.