HTTP Health Monitor in LTM

this knowledge article explain how to build send strings quite well: https://support.f5.com/csp/article/K2167

so you first send will end up something like this, where the Host part it optional

GET /Sample/healthchecksimple HTTP/1.1\r\nHost: host.domain.com\r\nConnection: Close\r\n\r\n

as for your receive what are trying to get exactly? HTTP/1.1 200 or OK ? specially those seem a bit odd, as they can easily cause a false up if OK is somewhere further.