anilalan can you describe what kind of platform you want to integrate login with?
While Flarum extensibility theoretically makes it possible to implement login with any system, only oauth-based login is fairly straightforward to implement. Other solutions including global logins of all sorts require good knowledge of Flarum internals.
The SSO extension can be a good starting point for a custom integration https://discuss.flarum.org/d/21666-php-and-wordpress-single-sign-on-sso-with-optional-jwt-addon
If you only want to create a custom login form and not actually integrate with an external system, you'll need to use javascript and integrate with Flarum CSRF protection. The POST /login endpoint only returns JSON so cannot be used as the target for a basic HTTP form.
