How to use GraphQL mutation with Flutter

I use Hasura cloud to deal with GraphQL and database,but there were no data inserted:(
This is a very simple example of the app:

class GraphQLConfiguration {

 static HttpLink httpLink = HttpLink(
 'hasura api link',
);

static ValueNotifier<GraphQLClient> client = ValueNotifier(
  GraphQLClient(
   cache: GraphQLCache(),
   link: httpLink,
 ),
);

static ValueNotifier<GraphQLClient> clientToQuery() {
  return client;
 }
}