@dan Thanks for this. You are bang on point.
- We had to make a call to FusionAuth to fetch the code(requestID)
 - We built a AuthN request that looked something like this:
 
var samlRequestTemplate = `<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns3:AuthnRequest xmlns="urn:oasis:names:tc:SAML:2.0:assertion" xmlns:ns2="http://www.w3.org/2000/09/xmldsig#" xmlns:ns3="urn:oasis:names:tc:SAML:2.0:protocol"
    xmlns:ns4= "http://www.w3.org/2001/04/xmlenc#" Version="2.0" ProviderName="${idpName}"  ID ="${CODE_FROM_FUSIONAUTH}"  IssueInstant ="{issueInstant}" 
    Destination="${idpEndpoint}" ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST" AssertionConsumerServiceURL="${acsUrl}">
    <Issuer>${issuer}</Issuer>
    <ns3:NameIDPolicy Format="${nameIdFormat}" AllowCreate="false" />
</ns3:AuthnRequest>`;
- Post this request to the Identity Provider.
 - Complete the login flow and catch the response from IdP. It has the code included as a InResponseTo attribute within the encoded samlResponse.
 - Call fusion to Complete the Identity Provider Login.
 
Then on it is the same flow as a normal Login API based login.
Cheers 