code: private String getQueryString(final String[] contactIds) {
          StringBuilder stringBuilder = new StringBuilder();
          stringBuilder.append("_method=PUT");
  
          for (String groupId : contactIds) {
              stringBuilder.append("&ids[]=").append(groupId);
          }
  
          return stringBuilder.toString();
      }
docs: Builds a query string to add contacts to a group. We're using the
  alternative "/foo?_method=PUT&ids[]=foo&ids[]=bar" format to send the
  contact IDs as GET params. Sending these in the request body would
  require a painful workaround, as sendPayload sends request bodies as
  JSON by default. See also:
  https://developers.messagebird.com/docs/alternatives.