Pass RelayState to SP

Can anyone assist with an iRule that will capture relaystate as a variable and then pass that to the SP. We have a unique situation with a Vendor that wants an iOS/Android app to perform SSO, but they do not support SP initiated IDP. Goal - User access the app and is redirected to IDP Big-IP, then the relaystate is captured and this is passed to the SP. I have done many searches on this forum, but I am not seeing a resolution to this problem or verify if this is even possible.

Thanks in advance for any help you can provide.