Adding headers at the request level

I understand that you can add headers at the client level (that get sent for every request) but how can I add a header at the
request level.

e.g. This code shows how DefaultRequestHeaders can be set, but what if I wanted to add a request level header for say a Correlation Id?
```
var graphClient = new GraphQLClient("https://api.github.com/graphql");
graphClient.DefaultRequestHeaders.Add("Authorization", $"bearer {gitHubAccessToken}");
//var test = await graphClient.GetIntrospectionQueryAsync();
var request = new GraphQLRequest
{
Query = @"query { viewer { login } }"
};
var test = await graphClient.PostAsync(request);
```