First of all you need to make sure that you have configured LDAP. The dropdown should be available in the login screen of the backend and in the user section under LDAP provider.
Furthermore, all configurations from the following article should be applied.
should be the address of your STS server. If you request the address you should get a line stating that this is a Security Token Service.
Furthermore, you need to make sure that IIS configurations for STS site are correct.
In IIS Manager, select the STS site.
In section IIS on the right, double-click Authentication.
Choose one of the following authentication types and set it in IIS:
- If all computers that are used to authenticate in Sitefinity are part of the domain, enable Windows Authentication and disable all others.
- If there are computers that are not part of the domain and that are used for authentication, enable Basic Authentication and disable all others. You could turn https on for this site to protect the transferred credentials.