Using `fetch()` with ajaxAuth in Grails

login.js

const formData = new FormData()
formData.append('username', 'foo@bar.com');
formData.append('password', 'password');
const data = new URLSearchParams(formData);

fetch('/login/authenticate', {
    method: "POST",
    headers: {
        "X-Requested-With": "XMLHttpRequest",
        "Content-Type": "application/x-www-form-urlencoded",
    },
    body: data
}).then(response => {
    response.json().then(data => {
        if(data.username) {
            console.log(data.username)
        } else {
            console.log(data.error)
        }
    })
}).catch(response => response.json().then(data => {
    console.log(data)
}));