Where are you authenticating user against? Are the users created into Adobe IMS through some batch process? One solution we implemented was using SAML over ADFS. So internal users are part of organization active directory. The homepage of react app running from AppBuilder is proxied at AEM Dispatcher. The Dispatcher sends the initial request to AEM Publisher. AEM has SAML configuration to connect to organization ADFS. So user is taken to office365 login screen. Once logged in, dispatcher proxies the request to AppBuilder and react app is served. We didn't build login screen, instead reused saml login.
If your user accounts are not in active directory, this wont work.