public URI getCallbackUrl() throws GuacamoleException {
        return environment.getRequiredProperty(SAML_CALLBACK_URL);
    }
The callback URL used for the SAML IdP to POST a response
to upon completion of authentication, normally the base
of the Guacamole install.

@return
    The callback URL to be sent to the SAML IdP that will
    be POSTed to upon completion of SAML authentication.

@throws GuacamoleException
    If guacamole.properties cannot be parsed, or the property
    is missing.
extensions/guacamole-auth-sso/modules/guacamole-auth-sso-saml/src/main/java/org/apache/guacamole/auth/saml/conf/ConfigurationService.java