To use a proxy with shoutrrr, you could either set the proxy URL in the environment variable HTTP_PROXY or override the default HTTP client like this:

proxyurl, err := url.Parse("socks5://localhost:1337")
if err != nil {
    log.Fatalf("Error parsing proxy URL: %q", err)
}

http.DefaultClient.Transport = &http.Transport{
    Proxy: http.ProxyURL(proxyurl),
    DialContext: (&net.Dialer{
        Timeout:   30 * time.Second,
        KeepAlive: 30 * time.Second,
    }).DialContext,
    ForceAttemptHTTP2:     true,
    MaxIdleConns:          100,
    IdleConnTimeout:       90 * time.Second,
    TLSHandshakeTimeout:   10 * time.Second,
    ExpectContinueTimeout: 1 * time.Second,
}