How do I transfer a http-header attribute as a URL parameter to an upstream?

Hi
How do I transfer a http-header attribute as a URL parameter to an upstream?

I am configuring a reverse proxy and i am stuck with a configuration problem.
I've configured a location "/". Request to / must contain a special header attribute. Now i must pass this attribute to an upstream but as URL parameter.

Is this possible at all?

Cheers
Stefan