Function direct_url 

pub fn direct_url(
    endpoint: &str,
    guest_port: u16,
    target: &str,
) -> Result<String, Error>

The URL a direct session’s request targets: the endpoint’s host at guest_port.

Through the proxy the guest port travels as a header; with no proxy in the path, the port is the port. The scheme is the endpoint’s, read the way upstream_url reads it.