Sample Federation URL:
I make this post, as much about a note to myself – as for others. This URL will avoid using the Alias service, and initiate a HTTPPost SAML flow.
https://<idpHost>/FIM/sps/<federationname>/saml20/logininitial?RequestBinding=HTTPPost&ResponseBinding=HTTPPost&NameIdFormat=Email&PartnerId=<ID for partner>
Here is some additional notes from the IBM Knowledge Center: