https/ssl standalone webserver in golang

Question

I tried to make a https/SSL standalone webserver in Golang. Any help with Acme DNS challenge, I have DNS server written in Go just, i just need that token from the ACME challenge so i can put it as TXT record in _acme-challenge.realwebsite.com

package main

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

func main() {
  mux := http.NewServeMux()
  mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    fmt.Fprint(w, "Hello HTTP/2")
  })

  server := http.Server{
    Addr:    ":443",
    Handler: mux,
    TLSConfig: &tls.Config{
      NextProtos: []string{"h2", "http/1.1"},
    },
  }

  fmt.Printf("Server listening on %s", server.Addr)
  if err := server.ListenAndServeTLS("certs/realwebsite.com.crt", "certs/realwebsite.com.key"); err != nil {
    fmt.Println(err)
  }
}