The current redirecting is a complex interplay between the blocking page (which is going to be obsoleted), a lighttpd config option and some PHP code in the AdminLTE package. In the future (the in-progress Pi-hole v6.0), things will become much easier to manage and I already added the requested feature here:
This makes Pi-hole v6.0 having an option WEBDOMAIN which can be set to any domain. If this domain is called, the redirecting to /admin will happen.