Host header doesn't match request-authority when using a proxy (RFC 7230#5.4)

[TobiasWildbrett]

Hi,

I'm facing a problem when trying to send a https request through a proxy using the HttpRequestMessage method in .NET Framework 4.8
The proxy is returning a code 400 bad request, because the value of the Host header in the CONNECT method does not match the request-target. Using curl I'm able to send the identical request.

Using system diagnostics I found that the header is missing the port number:

HttpWebRequest#33711845 - Request: CONNECT www.example.com:443 HTTP/1.1
ConnectStream#12547953 - Header
{
Host: www.example.com
Proxy-Connection: Keep-Alive
}

The proxy is an haproxy, which seems to be very strict in checking this header according to RFC 7230#5.4
Please also refer to the following issue haproxy/haproxy#1159

I tried several ways of working around this issue, like adding the Host header with port in the original request, but I couldn't find a way to modify the header for the CONNECT method.
Also the uri method is removing the port number if it is the default port for the used scheme.

Is there any other way to add the port number to the Header, according to RFC 7230#5.4?

Thanks.