Add query param to backend requests

[filex]

My upstream systems needs an authorization token to be passed as a query parameter. Let's call it token=….

I configure an /upstream/** endpoint and route all requests to a backend configuration. As of now, all client requests hitting /usptream will be passed unchanged to the backend (except for the path being changed by /**).

The token is an authorization mechanism that I want to implement in couper. Maybe I will extract it from a cookie, an arbitrary header or a JWT token. Now I am looking for a way to add it as a query parameter on every upstream request.