How to use before-body pandoc argument in R markdown?

Question

I want to insert a document before the table of contents of my Word exported markdown document.

My idea is to change pandoc template as indicated here.

Before changing the pandoc template however, I need to pass the document to insert to pandoc.

So I tried:

---
title: "Test"
author: "ben"
date: "2/2/2017"
output:
  word_document:
    toc: true
    pandoc_args: [
      "-B", "BeforeBody.Rmd"
    ]
---

```{r setup, include=FALSE}
knitr::opts_chunk$set(echo = TRUE)
```

# R Markdown

This is an R Markdown document. Markdown is a simple formatting syntax for authoring HTML, PDF, and MS Word documents. For more details on using R Markdown see <http://rmarkdown.rstudio.com>.

# Click

When you click the **Knit** button a document will be generated that includes both content as well as the output of any embedded R code chunks within the document. You can embed an R code chunk like this

But nothing happens, I just have my Word document, without the inserted text before the body.

I also tried to insert a Word document:

output:
  word_document:
    toc: true
    pandoc_args: [
      "-B", "/Users/ben/test.docx"
    ]

But nothing happens.

I also tried:

output:
  word_document:
    toc: true
    pandoc_args: [
      "--include-before-body", "/Users/ben/test.docx"
    ]

and

output:
  word_document:
    toc: true
    pandoc_args: [
      "--include-before", "/Users/ben/test.docx"
    ]

... still nothing. What can I do?