The Credential object is used to store all information needed to authenticate to Betfair. The same object can be reused by any client making a connection to Betfair. The Credential object has no publicly available methods. It is used exclusively by other internal processes in the library.
var credentials = new Credential([USERNAME], [PASSWORD], [APPKEY]);
It is recommended by Betfair that you use a certificate for non-interactive bot login. Learn more here.
To use a certificate to login:
var cert = X509Certificate2.CreateFrom...;
var credentials = new Credential([USERNAME], [PASSWORD], [APPKEY], cert);
You can view the Credential class here