How-to monitor pool member by FQDN vs IP

Do you mean the host header in the request ? if yes just create a monitor and add the host header with your fqdn in the send string.

If you mean the monitor should resolve the fqdn to ip before sending, we work in a slightly different way. you can starting with 11.6 define fqdn pool members, bigip will periodically resolve the fqdn to ip addresses, but your monitors will keep test the member ip address.

You can use something like this within the HTTP monitor "send string":

GET /index.html HTTP/1.1\r\nHost: host.domain.com\r\nConnection: Close\r\n

Reference: https://support.f5.com/kb/en-us/solutions/public/13000/300/sol13397.html