Directing the user to the admin panel

Hello, when a subsite is created and the user logs in to the subsite, it is redirected to example.com/subsite/wp-admin and directly redirected to the admin panel.

However, if we define a top-level domain name for this subsite (such as customdomain.com) and the user logs in from the main site (example.com/wp-admin), he will be redirected to customdomain.com/wp-admin and must log in again. However, the user still appears logged in to the main site. You can’t just access the site’s admin panel. To do this, he must log in again at customdomain.com/wp-admin.

What do we need to do to direct the user directly to customdomain.com/wp-admin and access the admin panel when he logs in to the main site?