If automated security scanners are unable to test your application's external endpoints, you should use a Dynamic Application Security Test (DAST) scanner. These scanners interact with web applications through their front end, simulating real-world attacks. Examples include OWASP ZAP, Burp Suite, VeraCode Dynamic Analysis, Intruder, and Acunetix. Make sure to obtain the necessary permissions to perform security testing on external endpoints owned by third parties.