XHR POST with FormData object sends as Request Payload instead of FormData

Question

I've read a handful of questions on Stack Overflow about this, but none seem to help.

I have a form, and I want to submit it. When I use a input type="submit", I can see in the headers the data goes up in the "Form Data" part of the header.

I then want to use XHR instead of the form's submit button.

I do:

var xhr = new XMLHttpRequest();
xhr.open("POST", "/url", true);
xhr.send(new FormData(document.getElementById("myform")));

Now in the header for this request, the data is there, but not in "Form Data", but in "Request Payload".

I think I can read the payload on my server (Java, Google App Engine) and parse it manually, I think. But that seems overkill.

Can I force the XHR to send the FormData as "Form Data" instead of "Request Payload"?