How to use ⁠client::conn::http2 to establish a via CONNECT and send partial data before returning the status.

[Orvaxis]

I’m trying to use an HTTP/2 connection as the foundation to implement a proxy. However, I want to send the proxy’s own request headers for protocol extension at the same time initiating the request. It seems that with Hyper, I can only obtain the Upgrade and send data after receiving the server’s OK response.
Can someone help me and tell me how I should do this?