Write Malformed JSON in the body Plug

Questions / Help
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4

my current state of plug

defmodule PogioApi.Plug.PrepareParse do
  import Plug.Conn
  @env Application.get_env(:application_api, :env)

  def init(opts) do
    opts
  end

  def call(conn, _opts) do
    %{method: method} = conn
    if method in ["POST"] and not(@env in [:test]) do
      {:ok, body, _conn} = Plug.Conn.read_body(conn)
      case Jason.decode(body) do
        {:ok, _result} ->
          private = Map.put(conn.private, :body, body)
          %{conn | private: private}
        {:error, _reason} ->
          error = %{message: "Malformed JSON in the body"}
          conn
          |> put_resp_header("content-type", "application/json; charset=utf-8")
          |> send_resp(400, Jason.encode!(error))
          |> halt
      end
    else
      conn
    end
  end
end

I am trying to put back the body I read but not successful. Reference: How do you put a request body in a Plug.Conn?