FormData doesn't include value of buttons
Because the call to FormData's constructor doesn't know that it was triggered by a click on a submit button (let alone which submit button it was), and because you only want the used submit button's values included, the submit button isn't included in the posted data. You can use FormData.append to include desired pair.
$('#myBtn').click(function(e) {
e.preventDefault();
var formElement = $(this).closest('form')[0];
var formData = new FormData(formElement);
formData.append("lol","cats");
var request = new XMLHttpRequest();
request.open("POST", "https://posttestserver.com/post.php");
request.send(formData);
}); <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
<form method="POST" action="https://posttestserver.com/post.php">
<input type="text" name="wtf" />
<button type="submit" id="myBtn">
Huh
</button>
</form>