public interface Wypok {
    @FormUrlEncoded
    @Headers("apisign: xxxxxxxxxxxxxxxxxxxx")
    @POST("/user/login/appkey,yyyyyyyyyyyyyyyy")
    Call<ResponseBody> listRepos(@Field("accountkey") String accountkey, @Field("login") String login);
}

public class Demo {
    public static void main(String[] args) throws IOException {
        Retrofit retrofit = new Retrofit.Builder()
                .addConverterFactory(JacksonConverterFactory.create())
                .baseUrl("http://a.wykop.pl")
                .build();

        Wypok wypok = retrofit.create(Wypok.class);
        System.out.println(wypok.listRepos("zzzzzzzzzzzzzz", "Roballo").execute().body().string());
    }
}