code: public void putCustomQueryParameter(String name, String value) {
          if (customQueryParameters == null) {
              customQueryParameters = new HashMap<String, List<String>>();
          }
          List<String> paramList = customQueryParameters.get(name);
          if (paramList == null) {
              paramList = new LinkedList<String>();
              customQueryParameters.put(name, paramList);
          }
          paramList.add(value);
      }
docs: Add a custom query parameter for the request. Since multiple values are allowed for the same
  query parameter, this method does NOT overwrite any existing parameter values in the request.
  <p>
  Any custom query parameters that are defined are used in the HTTP request to the AWS service.
  
  @param name
  The name of the query parameter
  @param value
  The value of the query parameter. Only the parameter name will be added in the URI
  if the value is set to null. For example, putCustomQueryParameter("param", null)
  will be serialized to "?param", while putCustomQueryParameter("param", "") will be
  serialized to "?param=".