A Basic Go Webserver Using Certmagic listening on the ports 8443 and 8080.

webserver3.go

package main

import (
	"log"
	"net/http"

	"github.com/mholt/certmagic"
)

func main() {
	mux := http.NewServeMux()

	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		w.Write([]byte("Hello World"))
	})

	magic := certmagic.New(certmagic.Config{
		CA:     certmagic.LetsEncryptStagingCA,
		Email:  "[email protected]",
		Agreed: true,
		//AltHTTPPort: 8080,
		AltTLSALPNPort: 8443,
	})

	err := magic.Manage([]string{"example.com"})
	if err != nil {
		log.Fatal(err)
	}

	s := http.Server{
		Addr:      "8443",
		Handler:   mux,
		TLSConfig: magic.TLSConfig(),
	}

	log.Println("listening...")
	go log.Fatal(http.ListenAndServe(":8080", magic.HTTPChallengeHandler(mux)))
	log.Fatal(s.ListenAndServeTLS("", ""))
}

not sure how this works given that certmagic.New has an arity of two

You are correct. I assume certmagic have changed their api since I wrote this sample.