iManager and NAM

We are trying to protect iManager as a protected resource by using NAM.
The proxy service should take the user to the iManager logon screen and auto-fill the cn value (a string value) for the user.

I have created a resource that protects the imanager web-server, and have set a redirection in an authorization policy.
I had applied a form-fill policy that would submit the login ID (cn), and left the Auto-Submit option unchecked.
I am not specifying any contracts under Authentication procedure, since it will just be a redirection with an autopopulation of the user's cn.
I get the following error when I try to access the url. I later disabled the form-fill policy but I get the same error.

Status: 504 Gateway Time-Out
Description: Unable to connect to origin web server. The web site you are attempting to access is currently unreachable. This may be due to a network outage, or the web site might be experiencing technical difficulties. If this condition persists, please contact the helpdesk


Does anyone have any thoughts on how this might be achieved i.e. NAM form-filling the cn value on iManager screen, without submitting the form?


Thanks,
Sadhani