https://goenning.net/2017/11/08/free-and-automated-ssl-certificates-with-go/

autocert.go

package main

import (
	"crypto/tls"
	"fmt"
	"net/http"

	"golang.org/x/crypto/acme/autocert"
)

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintf(w, "Hello Secure World")
	})

	certManager := autocert.Manager{
		Prompt: autocert.AcceptTOS,
		Cache:  autocert.DirCache("certs"),
	}

	server := &http.Server{
		Addr:    ":443",
		Handler: mux,
		TLSConfig: &tls.Config{
			GetCertificate: certManager.GetCertificate,
		},
	}

	go http.ListenAndServe(":80", certManager.HTTPHandler(nil))
	server.ListenAndServeTLS("", "")
}