You can authenticate with vCenter Server by using a user name and password known to the vCenter Single Sign-On service.
If you prefer to delegate the process of requesting a SAML token for your API client, you can present your vCenter Single Sign-On domain credentials to the vSphere Automation API endpoint and request a session ID. The endpoint process forwards your credentials to the vCenter Single Sign-On service and requests a SAML token on your behalf. In this case, you never deal with the token.