[Question] When using custom fetch for http link, how can we pass some parameter to the options argument?

[amadeu01]

Where does the options come from?

const customFetch = (uri, options) => {
  const { operationName } = JSON.parse(options.body);
  return fetch(`${uri}/graph/graphql?opname=${operationName}`, options);
};

const link = createHttpLink({ fetch: customFetch });

The above example can be found at the documentation. But, it is not clear how can I change the value of the option from the query? How does the query impact over the option argument?