You should be able to add the relaystate as a query parameter in your request and that should configure the SAMLRequest.
I'm not sure what you're requesting in particular "post variable along with the samlrequest". Are you requesting the AAD server to send another post request with the relaystate in the body of the post to your server?
See here for more info on that : https://social.msdn.microsoft.com/Forums/azure/en-US/e6f5d9ee-9ca5-4027-971d-b89735fe2a85/does-azure-support-dynamic-relaystate