This worked for me
var serializeJSON = function(data) {
return Object.keys(data).map(function (keyName) {
return encodeURIComponent(keyName) + '=' + encodeURIComponent(data[keyName])
}).join('&');
}
var response = fetch(url, {
method: 'POST',
body: serializeJSON({
haha: 'input'
})
});