jQuery Ajax POST失败

I can't get my AJAX request to complete successfully. It's for the bearere token on Twitter's API. I have done successfully in a HTTP request client as shown below:

But I can't replicate this in code:

$.ajax({
        type: 'POST',
        url: 'https://api.twitter.com/oauth2/token',
        headers: {
            'Authorization':'Basic ' + base64EncodedBearerTokenCredentials,
        },
        contentType: 'application/x-www-form-urlencoded; charset=utf-8',
        data: {
            'grant_type': 'client_credentials'
        },
        error: function(xhr, status, error) {
        },
        success: function(response){
        },
    });

I'm getting a 405, but because I have the Authorization header in I can't see the response in Firebug.

What is it?

Thanks.