The URL cynic.symantec.com is no longer valid.
The earliest versions of Symantec Advanced Threat Protection (ATP) (now Symantec Endpoint Detection and Response) did not have the ability to submit files to Symantec's Cynic server for sandbox analysis. So users could manually submit files for analysis through the Cynic portal.
Symantec Endpoint Detection and Response lets users submit files for sandbox analysis through the EDR appliance console. Symantec has deprecated the Cynic portal.