How To Send JSON & Set Header Request On Net.Httppost() Method?

Hi everyone, I'm using Net.hTTPPost(url, postData) method on ARC script (JavaScript). I've just realized that this method only send the body payload request as x-www-form-urlencoded. My problems are:

1. How to send JSON data? I've tried JSON.stringify() but it didn't work as my expectation.
2. How to set its header request? Here I attach my basic code to make a simple POST request to JSON Placeholder Fake API. I'm curious how to send JSON & how to set its header request.

var resPost = Net.hTTPPost(
   "https://jsonplaceholder.typicode.com/posts";,
   'name=Andy&age=20'
)
print(resPost)

resPost = JSON.parse(resPost)
print(resPost.name)
print(resPost.age)

Run this code & you will see the following log in console:

> {
  "name": "Andy",
  "age": "20",
  "id": 101
}
> Andy
> 20

Hope someone in this community can help me solve this problem. Thanks in advance.