Persist based on query string

Your iRule seems to be just performing a load balancing decision and not persisting. So, you can use something like this:

when HTTP_REQUEST { 
 If the uri contains a query parameter named server
if { [HTTP::uri] contains "server" } {
     Define a lowercase variable to store the server name
    set webserver [URI::query [HTTP::uri]]
     If the server query parameter matches an entry in the datagroup
    if { [class match $webserver equals servername2ip_datagroup] } {
        pool POOL_WITH_NODE
        }
    }
}