How haproxy httpchk request add Host headers


We use haproxy in our project, and now we use haproxy to do the health check, and we want the haproxy health check request has Host headers, but should not specify the specific host names, that means: if we have two backend servers,
the health check request to each one with the host header of its self ip address. I see the document that the host filed is fixed like below:

backend https_relay
mode tcp
option httpchk OPTIONS * HTTP/1.1\r\nHost:\ www
server apache1 check port 80

How can we use the backend server ip address as host field in haproxy health check request and not fixed field as there may be 3 or 4 backend servers. How haproxy support this feature?

Thanks for your help.