Thank you, Jason. I will give this a try, per your instructions.
Just a thought, if the user session is successfully passed onto the other domain, will the user be able to access content which has SA access rules applied to them? The other domain that the form posts to includes several pages where SA access rules have been applied - where a specific userLevel is required to access such content. In this case, the UserID that is being passed is associated with a recognized userLevel.