Adds an SSO URL to the list.
Use this method to add a new single sign-on (SSO) link. The method returns the index of the new link in the internal list of SSO links.
SPIndex specifies the index of the service provider configuration, URL contains the relative URL for the SSO, and RelayState contains the value of the corresponding SAML parameter.
According to SAML 2.0 specification, the RelayState parameter may be used by the SAML binding in order to convey and preserve state information. If this parameter is present in a SAML request message, the responding party must include the same exact parameter into the response to this request.
In many applications, when using IdP-initiated single sign-on, the IdP uses RelayState to indicate to the SP the URL to which it should redirect after the successful sign-on.