you say "what will be the hostname when traffic is sent to backend servers?".
do you mean the traffic from the health monitor to the backend server?
it won't contain a hostname in that traffic, you didn't provide one in the Send String so the BIG-IP won't make on up.
this is quite easy to check if you have just one health monitor and no live traffic yet, run tcpdump on the BIG-IP and use something like this:
tcpdump -i 0.0 -nn host <backend server IP> -c 10 -X
you will see the Send String somewhere without any Host: header.