How to specify a pool member based on Headers?

You can technically perform pool member selection on any request criteria. Example:

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    switch [string tolower [HTTP::header POOLSELECT]] {
        "member1" { pool $default_pool member 10.200.0.1 80 }
        "member2" { pool $default_pool member 10.200.0.2 80 }
        "member3" { pool $default_pool member 10.200.0.3 80 }
    }
}

where POOLSELECT is an arbitrary that you must inject with an arbitrary value. You may find, however, that injecting an HTTP header with a browser would be challenging. So an easier option may be to use the HTTP URI, or specifically a query string. Example:

when CLIENT_ACCEPTED {
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    if { [string tolower [HTTP::uri]] contains "nodeselect=" } {
        switch [string tolower [URI::query [HTTP::uri] nodeselect]] {
            "member1" { pool $default_pool member 10.200.0.1 80 }
            "member2" { pool $default_pool member 10.200.0.2 80 }
            "member3" { pool $default_pool member 10.200.0.3 80 }
        }
    }
}

where nodeselect is a query string parameter like this:

http://www.example.com/foo/bar?nodeselect=member1