如何在retroift POST请求中添加常量正文字段？

@POST("api/endpoint")
Call<ResponseBody> postData(@Body RequestBody requestBody);

MediaType mediaType = MediaType.parse("application/json");
RequestBody requestBody = RequestBody.create(mediaType, "{\"key\":\"value\"}");

Request.Builder requestBuilder = new Request.Builder()
        .url("https://api.example.com/api/endpoint")
        .post(requestBody);

FormBody.Builder formBodyBuilder = new FormBody.Builder()
        .add("constantKey", "constantValue");

requestBuilder.post(formBodyBuilder.build());

Call<ResponseBody> call = retrofitService.postData(requestBuilder.build());
call.enqueue(new Callback<ResponseBody>() {
    // 处理请求结果
});