Post request JSON in body with Retrofit 2.0

I want to send a JSON parameter to an API, and what I have achieved was like so :

{"v1" : "username", "v2" : "password"}

So basically I am sending 2 JSON object with "v1" and "v2" as the parameter. But what I wanted to achieve is sending the parameter like so :

{"username" : "password"}

I couldn't figure out how to do this. Here is my code for now :

POJO Class

class Post {

    private String v1;
    private String v2;
    private PostSuccess SUCCESS;

    public Post(String name, String password) {
        this.v1 = name;
        this.v2 = password;
    }
}

class PostSuccess {
    @SerializedName("200")
    private String resp;
    private String result;

    public String getResp() {
        return resp;
    }

    public String getResult() {
        return result;
    }
}

POST Interface

public interface JsonPlaceHolderApi {
    @POST("ratec")
    Call<Post> createPost(@Body Post post);
}

MainActivity Class

private void createPost() {
        final Post post = new Post("anthony", "21.000008", "72", "2");
        Call<Post> call = jsonPlaceHolderApi.createPost(post);

        call.enqueue(new Callback<Post>() {
            @Override
            public void onResponse(Call<Post> call, Response<Post> response) {
                if (!response.isSuccessful()) {
                    textViewResult.setText("Code: " + response.code());
                    return;
                }    

                Post postResponse = response.body();
                String content = "";
                content += "Code : " + response.code() + "\n";
                textViewResult.setText(content);

            }
            @Override
            public void onFailure(Call<Post> call, Throwable t) {
                textViewResult.setText(t.getMessage());
            }
        });
    }

As you can see, this is the parameter that I am sending :

final Post post = new Post("name", "password");
    Call<Post> call = jsonPlaceHolderApi.createPost(post);

And in the POJO class, I have declared "v1" and "v2", so instead of sending this :

{"username" : "password"}

I am sending this :

{"v1" : "username", "v2" : "password"}

I appreciate your help and suggestions. Thanks!

You can directly use the map in the @Body and access the key and value of the map as below:

public interface JsonPlaceHolderApi {
    @POST("ratec")
    Call<Post> createPost(@Body Map<String,String> post);
}