Headers authorization not added to requests by customrequestheaders

Hi I'm trying to pass an authorization header for login in another domain (backend) with the same basic authentication, but the header is not added to requests

labels:
      - traefik.http.middlewares.ui-auth.basicauth.users=${BASIC_AUTH}
      - traefik.http.middlewares.ui-header.headers.customrequestheaders.authorization=Basic encod(user:pass)
      - traefik.http.middlewares.ui-header.headers.accessControlAllowCredentials=true
      - traefik.http.routers.prefectui.middlewares=ui-auth,ui-header@docker

Where could i go wrong? Thanks