encoder.exs

defmodule Encoder do
  def encode(
        username,
        msg,
        id \\ "mguFd-51",
        to \\ "79040883-331c-4944-aca5-16e9351098c1@localhost"
      ) do
    {:xmlel, "message",
     [
       {"xml:lang", "en"},
       {"to", to},
       {"id", id},
       {"type", "chat"}
     ],
     [
       {:xmlel, "body", [], [xmlcdata: msg]},
       {:xmlel, "username", [{"xmlns", "message:username"}], [xmlcdata: username]},
       {:xmlel, "type", [{"xmlns", "message:type"}], [xmlcdata: "text"]}
     ]}
  end
end

[id, username, to, msg] = System.argv()

msg =
  Encoder.encode(username, msg, id, to)
  |> :erlang.term_to_binary()
  |> Base.encode16()

# Это просто распечатывает результат
"0x" <> msg
|> IO.puts()