Caddy equivalent of nginx's proxy_pass_header

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I’m running both Caddy and Transmission on Ubuntu server 16.04.

Caddy is on version 0.10.4 and Transmission is on version transmission-daemon 2.92 (14714)

This is my caddy.conf file:

#CHRISKOSTER.NL

www.chriskoster.nl {
    redir https://chriskoster.nl{uri}
}

chriskoster.nl {
    root /home/chris/web/chriskoster.nl

    log /home/chris/web/log/chriskoster.nl.access.log {
        rotate_size 100  # Rotate after 100 MB
        rotate_age  14   # Keep log files for 14 days
        rotate_keep 10   # keep maximum of 10 lof files
    }

# not working yet
    proxy /tvheadend localhost:9981 {
        transparent
    }

# not working yet
    proxy /transmission localhost:9091/transmission/web/ {
        #header_upstream X-Transmission-Session-Id {>X-Transmission-Session-Id}
        transparent
    }

    errors /home/chris/web/log/chriskoster.nl.error.log

    gzip

}

I start caddy using systemd service, and transmission with init.d sctipt but both are under the same user. This is my start line for caddy:

ExecStart=/usr/local/bin/caddy -quic -log stdout -agree=true -conf=/home/chris/web/conf/caddy.conf

When I search for transmission behin nginx, the first results in Google are some gists whith different nginx configs, most of them are more about enabling ssl in the proxy and the path is just server:port