The data is not proper JSON, change it to:

let data = {"article_id": id};

And make sure you encode it:

JSON.stringify(data)

My backend API accepts data in JSON format, such as:

{ "article_id" = 1 }

In the front-end, I tried to add the following javascript to a button:

function articleIsSelected(id) {
  let data = '{"article_id":' + id + '}';

  $.ajax({
    url:"https://www.myurl.com",
    data: data,
    type: "post",
    contentType: "application/json",

    success: function () {
      alert("Selection succeeded!");
    },
    error: function () {
      alert("Selection failed.");
    },
  });
}

It returns that the request was successful, but my database is not updated. There is something wrong with the data format. Instead of trying to hard code the data in JSON format, one should sign the value to "article_id" and then JSON encode it with JSON.stringify(data).