How to use other headers as variables in customRequestHeaders

Traefik Traefik v2
1

I would like to build X-Original-* headers from X-Forwarded-* headers
I thought I could do something like this:

apiVersion: traefik.containo.us/v1alpha1

kind: Middleware

metadata:

  name: x-original-headers

spec:

  headers:

    customRequestHeaders:

      X-Original-Method: $X-Forwarded-Method

      X-Original-URI: $X-Forwarded-Uri

      X-Original-URL: $Referer||$X-Forwarded-Uri

But it won't work. Does anyone know how to achieve this?
I didn't see any examples like this in documents.
Is using variable not supported in customRequestHeaders?
My traefik version is 2.2.11.

Thanks!