Programmatically capture and pass the same Azure ACS FedAuth key

Programmatically capture and pass the same Azure ACS FedAuth key for an ACS authenticated user to both SharePoint Online and SharePoint on-prem when both are registered ‘Relying Parties’ to the same Azure ACS ‘Identity Provider’. 

Already able to capture the FedAuth Key from ACS and pass to both SharePoint deployments via ‘ExecutingWebRequest’, but the SharePoint deployments are rejecting the FedAuth key with a 403-Forbidden.  IE is able to pass the proper FedAuth across browser windows, but not able to capture and pass programmatically within an App for SharePoint.

Thoughts?