code: func (sp *ServiceProvider) MakePostAuthenticationRequest(relayState string) ([]byte, error) {
  	req, err := sp.MakeAuthenticationRequest(sp.GetSSOBindingLocation(HTTPPostBinding))
  	if err != nil {
  		return nil, err
  	}
  	return req.Post(relayState), nil
  }
docs: // MakePostAuthenticationRequest creates a SAML authentication request using
  // the HTTP-POST binding. It returns HTML text representing an HTML form that
  // can be sent presented to a browser to initiate the login process.