Correctly posting data with authentication in JS / JQuery

Hi all and happy easter,

I am writing an application through Electron for a simple text editor that posts drafts to medium.com. They provide an api and the documentation for it, but my knowledge in jquery and js is still a little limited. Essentially, I'm using Ajax to post the data to medium, but receiving a 400 error. I'm sure it's something really dumb and simple, but I cant figure it out, so here's the code i've written to post the data:

$('.save_draft').click(function(){

    var accessToken = 'xxxxx';

    $.ajax({
        url: "https://api.medium.com/v1/users/" + user.id + "/posts",
        type: 'POST',
        dataType: 'html',
        contentType: "application/json",
        beforeSend: function(xhr) {
            xhr.setRequestHeader("Authentication", accessToken)
        },
        data: {
            "title": "Liverpool FC",
            "contentFormat": "html",
            "content": "<h1>Liverpool FC</h1><p>You’ll never walk alone.</p>",
            "canonicalUrl": "http://jamietalbot.com/posts/liverpool-fc",
            "tags": ["football", "sport", "Liverpool"],
            "publishStatus": "draft",
        },
        success: function(data){
            alert(data);
        }
    });

});

Now i'm providing the accessToken, i've just 'xxxxx'd it for posting. user.id is received at the start, I can confirm that it's coming through correctly. As for the documentation provided, you can see it here: https://github.com/Medium/medium-api-docs#33-posts but essentially it's asking for this:

POST /v1/users/5303d74c64f66366f00cb9b2a94f3251bf5/posts HTTP/1.1
Host: api.medium.com
Authorization: Bearer 181d415f34379af07b2c11d144dfbe35d
Content-Type: application/json
Accept: application/json
Accept-Charset: utf-8
{
 "title": "Liverpool FC",
 "contentFormat": "html",
 "content": "<h1>Liverpool FC</h1><p>You’ll never walk alone.</p>",
 "canonicalUrl": "http://jamietalbot.com/posts/liverpool-fc",
 "tags": ["football", "sport", "Liverpool"],
 "publishStatus": "public"
}

Like I say, i'm somewhat inexperienced with headers in ajax and a little confused so any help would be appreciated. Thanks.

The updated code:

$.ajax({
        url: "https://api.medium.com:443/v1/users/" + user.data.id + "/posts",
        type: 'POST',
        headers: {
            'Authorization': 'Bearer ' + accessToken,
            'Content-Type': 'application/json',
            'Accept': 'application/json'
        },
        data: JSON.stringify({
            "title": "Liverpool FC",
            "contentFormat": "html",
            "content": "<h1>Liverpool FC</h1><p>You’ll never walk alone.</p>",
            "canonicalUrl": "http://jamietalbot.com/posts/liverpool-fc",
            "tags": ["football", "sport", "Liverpool"],
            "publishStatus": "draft",
        }),
        success: function(data){
            alert(data);
        }
    });