@dhait Hmmm. This issue is due to the OAuth2 specification. If we provided a REST endpoint, it wouldn't be OAuth/OIDC.
If you want to fully customize the look and feel of the login and registration experience, but not use the FusionAuth hosted login pages, the recommended solution is to code directly against our REST APIs:
https://fusionauth.io/docs/v1/tech/apis/login
https://fusionauth.io/docs/v1/tech/apis/registrations
Does this solve your problem?