Cognos Security: Set-up/ pass Dummy User credential in Cognos as a macro

[Cognos91]

I have a requirement to pass or may set-up a dummy user macro in Cognos (or FM) so that if the user does not exist or has not been set-up yet, then, even if the user click on Cognos Reporting link, he/ she should not be able to view the landing page.
A brief background -
LDAP is configured with the internal company website. A Cognos reporting link is provided within this site, so that when users click on this link, the Cognos window will open up (either embedded within the site) or as a new tab.
Currently, there is also an internal authentication tool that is being leveraged to authenticate user id in Cognos. This tool pings the internal site to confirm the user credentials, and once confirmed, the Cognos window will display the needed folders/ reports based on user security.

The requirement is - Isthere be a way to pass a default user ID so that when an unknown user, who has not been set up in LDAP, clicks on the reporting link in the site (or even logs via the Cognos URL), he or she will not be prompted to enter user credentials and see a default page in Cognos. I have attached a graphical representation of the current set-up. LDAP is not configured in Cognos.

Has anyone use this kind of set-up where a Cognos link is provided from an internal site? If so, then was LDAP configured in Cognos and is it mandatory?
Any inputs will be appreciated.

Thanks.