iRule to log a specific field in POST data

Please try this:

when HTTP_REQUEST {
    if { ( [string tolower [HTTP::uri]] equals "/loginform.html" ) and ( [HTTP::method] equals "POST" ) } {
        HTTP::collect [HTTP::header Content-Length]
    }
}
when HTTP_REQUEST_DATA {
    set username "unknown"
    foreach x [split [string tolower [HTTP::payload]] "&"] {
        if { $x starts_with "username=" } {
            set username [lindex [split $x "="] 1]
        }
    }
    log local0. "User $username attempted login from [IP::client_addr]:[TCP::client_port]"
}