This isn't supported by the SAML specification.
To achieve SAML SSO, the service provider must redirect to the identity provider. This is done by sending a SAML authn request either using an HTTP Redirect or HTTP Post (via the browser).
The SAML authn request may include the user's name and we support sending this but many identity providers don't support receiving the user name and will ignore it.
You cannot send the user's password for security reasons.
Login must occur at the identity provider site.
This isn't a limitation in our product but rather good security practice imposed by the SAML specification.