How do I post form data with fetch api?

My code:

fetch("api/xxx", {
    body: new FormData(document.getElementById("form")),
    headers: {
        "Content-Type": "application/x-www-form-urlencoded",
        // "Content-Type": "multipart/form-data",
    },
    method: "post",
}

I tried to post my form using fetch api, and the body it sends is like:

-----------------------------114782935826962
Content-Disposition: form-data; name="email"

test@example.com
-----------------------------114782935826962
Content-Disposition: form-data; name="password"

pw
-----------------------------114782935826962--

(I don’t know why the number in boundary is changed every time it sends…)

I would like it to send the data with “Content-Type”: “application/x-www-form-urlencoded”, what should I do? Or if I just have to deal with it, how do I decode the data in my controller?

To whom answer my question, I know I can do it with:

fetch("api/xxx", {
    body: "email=test@example.com&password=pw",
    headers: {
        "Content-Type": "application/x-www-form-urlencoded",
    },
    method: "post",
}

What I want is something like $(“#form”).serialize() in jQuery (w/o using jQuery) or the way to decode mulitpart/form-data in controller. Thanks for your answers though.