Request user input via a form before proceeding with a login to an external Identity Provider

For example, ask user for this extra input after clicking on the IdP’s login button (and just before method performLogin is called). In fact, we have a federation of thousands of endpoints and the main idea is to define one and only SAML Identity Provider that allows user to select his company’s IdP though a form just before proceeding with the flow already implemented into SAMLIdentityProvider.