according to attached screenshot SSL certificate is invalid for mentioned API. Is it self signed? If yes then you may add this line of code:
#if DEBUG
ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
#endif
NOTE: you should NOT use this code in prod because it will avoid errors with real certificate which should not be avoided (that's why I added #if DEBUG here - this code will work only with Debug configuration).