Setting custom headers for individual requests

I’d like to set custom headers for individual requests, e.g. not when initialising ApolloClient.

I am running a query like this:

client.query({ query: /* some graphql query */});

I have tried like this:

client.query({
	query: /* some graphql query */,
	contenxt: {
		headers: {
			"SomeCustomHeader": 123
		}
	}
});

But that did not work. Is there another way to do it, on the fly?

I have found this, but it does not work: Advanced HTTP networking - Client (React) - Apollo GraphQL Docs