HealthCheck passing dynamic attributes

I have a requirement where I need to do health check and the exposed
health check api requires a uniqueId header, How can i set this header
to a different unique value in haproxy.cfg file . My configuration is as

backend server_1
mode http
balance roundrobin
option httpchk GET /health HTTP/1.0\r\nuniqueId:\
http-check expect status 200
server app1 check