How to pass path from haproxy frontend to backend in version 1.5.18

Question

I have a condition where I need to pass whatever path is given in the frontend url to a backend redirect statement. E.g. Frontend https://example.com/abc/xyz to backend http://server-address:port/abc/xyz. I am using http redirect in the backend as of now. But now it has been requested to pass any path along with the frontend URL, to be appended to backend as well. I cannot use redirect on frontend as I need to use port 443 which is being used by other service. So I am use acl_path to point to a different backend, so I need to achieve this using backend itself. My frontend config is below-

  frontend a
  bind 10.10.10.10:443 ssl  crt /etc/pki/org/key/somekey.key force-tlsv12 ciphers SOME-CIPHER
  mode http
  acl a_dev_app hdr_dom(host) -m beg a.dev.app.
  acl b_dev_app hdr_dom(host) -m beg b.dev.app.
  option http-server-close
  use_backend a if a_dev_app
  use_backend b if b_dev_app

Backend config-

backend b
  balance roundrobin
  cookie a-web-backend insert indirect nocache
  http-request redirect location http://server-url:8081/b
  mode http
  option httpchk /b
  server a-web-d01 http://server-url:8081 check maxconn 150 cookie a-web-d01