Currently the Surround SCM API does not support logging in via single sign-on, so you must specify a username and password to use the API.
It sounds like your administrator has configured Surround SCM to require single sign-on. If you go to the Server Options dialog in the Seapine License Server Admin Utility, select your LDAP or AD server, and press the Edit button, then you can see the single sign-on settings. The administrator can choose Enabled, Disabled, or Required. In your situation, it sounds like Required is configured instead of Enabled.
Depending on what you are using the C++ application for, you might want to consider a dedicated user account for these types of applications. That allows you to restrict the security access for that username to only operations that you want the automated process to perform. Any write operations would also be identified as being made by that special user. Again it depends on the goal of the C++ application.