Does anyone have an example on how to add more req.headers["whateverX"] to a WebSocket?

This is the vibe.d req.headers, and I want to append some more from within my subscribeWebSocket method.
auto res = requestHTTP(url, (scope req){

req.method = HTTPMethod.GET;
req.headers["Upgrade"] = "websocket";
req.headers["Connection"] = "Upgrade";
req.headers["Sec-WebSocket-Version"] = "13";
req.headers["Sec-WebSocket-Key"] = challengeKey;


WebSocket subscribeWebSocket()

auto ws_url = URL("wss://example.com/api/ws");
auto ws = connectWebSocket(ws_url);
return ws;


Thanks in advance.