How to use OKHTTP to make a post request?

As per the docs, OkHttp version 3 replaced FormEncodingBuilder with FormBody and FormBody.Builder(), so the old examples won't work anymore.

Form and Multipart bodies are now modeled. We've replaced the opaque FormEncodingBuilder with the more powerful FormBody and FormBody.Builder combo.

Similarly we've upgraded MultipartBuilder into MultipartBody, MultipartBody.Part, and MultipartBody.Builder.

So if you're using OkHttp 3.x try the following example:

OkHttpClient client = new OkHttpClient();

RequestBody formBody = new FormBody.Builder()
        .add("message", "Your message")
        .build();
Request request = new Request.Builder()
        .url("https://www.example.com/index.php")
        .post(formBody)
        .build();

try {
    Response response = client.newCall(request).execute();

    // Do something with the response.
} catch (IOException e) {
    e.printStackTrace();
}

The current accepted answer is out of date. Now if you want to create a post request and add parameters to it you should user MultipartBody.Builder as Mime Craft now is deprecated.

RequestBody requestBody = new MultipartBody.Builder()
        .setType(MultipartBody.FORM)
        .addFormDataPart("somParam", "someValue")
        .build();

Request request = new Request.Builder()
        .url(BASE_URL + route)
        .post(requestBody)
        .build();