Unable to add `x-stream-ext` header for before send webhook

Per the [docs](https://getstream.io/chat/docs/go-golang/before_message_send_webhook/#request-info), I should be able to set an `x-stream-ext` header to pass arbitrary info through the before send webhook's `request_info` payload, but I'm unable to find how to add configure that. Is this not supported in the Go SDK?