authentik/website/docs/add-secure-apps/providers/proxy/_caddy_standalone.md

Use the following configuration:

app.company {
    # directive execution order is only as stated if enclosed with route.
    route {
        # always forward outpost path to actual outpost
        reverse_proxy /outpost.goauthentik.io/* http://outpost.company:9000

        # forward authentication to outpost
        forward_auth http://outpost.company:9000 {
            uri /outpost.goauthentik.io/auth/caddy

            # capitalization of the headers is important, otherwise they will be empty
            copy_headers X-Authentik-Username X-Authentik-Groups X-Authentik-Email X-Authentik-Name X-Authentik-Uid X-Authentik-Jwt X-Authentik-Meta-Jwks X-Authentik-Meta-Outpost X-Authentik-Meta-Provider X-Authentik-Meta-App X-Authentik-Meta-Version

            # optional, in this config trust all private ranges, should probably be set to the outposts IP
            trusted_proxies private_ranges
        }

        # actual site configuration below, for example
        reverse_proxy localhost:1234
    }
}

If you're trying to proxy to an upstream over HTTPS, you need to set the Host header to the value they expect for it to work correctly.

reverse_proxy /outpost.goauthentik.io/* https://outpost.company {
    header_up Host {http.reverse_proxy.upstream.hostport}
}