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I believe the following could be your solution but you need to make sure you have a pool configured named POOL-sap10-8445 with the appropriate pool members in it listening on port 8445 this way you don't have to feed a port location in the HOST field of the HTTP request.

when CLIENT_ACCEPTED {

    set DEFAULT_POOL [LB::server pool]

}

when HTTP_REQUEST {

    set HOST [string tolower [HTTP::host]]
    set URI [string tolower [HTTP::uri]]

    if { (($HOST eq "forex.mycomp.com") and ($URI eq "/forex")) } {

        set SERVER_HOST [string map -nocase { "forex.mycomp.com" "sap10.company.comany.de"} [HTTP::host]]
        
        HTTP::host $SERVER_HOST
        pool POOL_sap10_8445

    } else {
        $DEFAULT_POOL
    }

}

 This configuration also assumes that you have a default pool configured on your virtual server so that any request not matching what you are concerned with goes to that pool.