IdP not maintaining POST params from one SP to another

[Brian Reindel]

We have cross linking across service providers, but as a part of that
cross link we're not issuing a simple GET. It is a POST with several
parameters in the POST body. The first time around if the user has not
established a session on the SP you're linking to this will not work
(params not maintained and GET issued after redirect). However, the
second time around after the session has been established it works
just fine. Are there configuration changes I can make to support this
functionality consistently?