I'm studying javascript methods but I have difficult to make POST method works. Below you can see the code that I use to test:

<form id="form_test">
    <h5>POST Method</h5>
    <input type="text" name="text" id="text_input">
    <button type="button" onclick="Save()">Send</button>
</form>

function Save() {
  var xhr = new XMLHttpRequest();
  var url = "/Page1?handler=Save";
  xhr.open("POST", url, true);

  let data = `{
        "Id": 78912,
        "Customer": "Jason Sweet",
        "Quantity": 1,
        "Price": 18.00
      }`;

  xhr.send(data);
}

public IActionResult OnPostSave()
        {
            return new JsonResult("my result");
        }

Any idea what can do wrong?