How do I AJAX POST to my NodeJS server?

I'm just trying to receive data in my server from ajax post.

My server receives the request normally but the req.body is an an empty object {}.Normaly I would send an object from JS looking like this {username: "holiday"}. I can't find a way to send this via AJAX construct. It must be possible...

Edit:

Setting header "Content-Type" to "application/json" allows data to be send via req.body, but I can't figure out how to correctly send the data from above.

I used the "Authorization" header to send the username with, which works, but far from ideal...