What’s the format of the body? Seems like you want application/x-www-form-urlencoded, so time to use Tesla.Middleware.FormUrlencoded, which uses URI.encode_query/1 under the hood.

    client = Tesla.client([
      {Tesla.Middleware.BaseUrl, "https://httpbin.org/"},
      Tesla.Middleware.EncodeFormUrlencoded,
      Tesla.Middleware.DecodeJson,
      Tesla.Middleware.Logger
    ])
    
    client |> Tesla.post("/post", [{"id[]", "1"},{"id[]", "2"}])

Output excerpt:

>>> REQUEST >>>
(no query)
content-type: application/x-www-form-urlencoded

id%5B%5D=1&id%5B%5D=2

<<< RESPONSE <<<

{
  "args": {},
  "data": "",
  "files": {},
  "form": {
    "id[]": [
      "1",
      "2"
    ]
  },
  "headers": {
    "Content-Length": "21",
    "Content-Type": "application/x-www-form-urlencoded",
    "Host": "httpbin.org",
  },
  "json": null,
  "url": "https://httpbin.org/post"
}

Please note

• [] notation to take values into array is not a standard but just common convention. e.g. See Plug.Conn.Query
• Elixir’s URI.decode_query returns a map, which is convenient but it’s wrong to use parse a string in urlencoded form, since WHATWG defines it as list of key/value, not map!

For that exact reason I wrote whatwg/www_form_urlencoded.ex at main · chulkilee/whatwg · GitHub a while ago.