Rewrite URL

I believe what you are looking for is a re-write, not a redirect. Something like:

Condition: http-host host equals a.domainname.com Action:http-uri request replace value: https://newwebservice/Custom/Uri

Updated for pool selection and query string preservation:

ltm policy DV {
    controls { forwarding }
    requires { http }
    rules {
        WebService {
            actions {
                0 {
                    http-uri
                    replace
                    query-string "tcl:[URI::query [HTTP::uri]]"
                    value https://newwebservice/Custom/Uri
                }
                1 {
                    forward
                    select
                    pool newwebservice_pool
                }
            }
            conditions {
                0 {
                    http-host
                    host
                    values { a.domainname.com }
                }
            }
            ordinal 1
        }
    }
    strategy first-match
}