On ThinRDP manager "Security" tab you will find the following options:
Authentication
Choose the level of authentication for the users access to ThinRDP. Users will still need to authenticate afterwards against the computer they connect to.
None
No authentication for ThinRDP access. This is only recommended for exclusive local access.
User / Password
Set your own credentials for ThinRDP access authentication.
Access Profiles
Manage the authentication with Active Directory users by creating a profile. Also select this option to enable profiles and set predetermined preferences for the ThinRDP users.
Use Standard browser authentication dialog
This option appears when "Authentication" is set to "Access Profiles". Check it to use the standard browser authentication dialog.
Manage Certificate
Press this button to access the options for replacing the default certificate installed with ThinRDP with your own.
Always remember to press "Apply" in order to save the changes.