automatic authentication

Hi everyone.
We have Active Directory in our office. ServiceDesk plus is installed and connected to Active Directory in admin setup settings. Also i choose "Enable Active Directory Authentication" and "Enable "Pass-through" Authentication" chekboxes.
When users try open SD page in browser they see table with authentication request. It's not a SD login screen, but brouther screen.
If they type login and pwd everything is OK and they go through in to SD home page without SD login screen.

On the comment under this setting i see note "On enabling "Pass-through" Authentication, ServiceDesk plus directly authenticates your windows system user name and password. Hence you need not login over again to enter ServiceDesk plus "

But why users should type login and password any time they open SD?

PS. if in admin settings in LDAP tab i'll choose "

Enable LDAP Authentication" i'll have the similar problem, but with authorization in SD system by login screen.

How can i automate all authentications to let all users in our AD to login with their AD accounts?
Please help me!