Submitting multiple forms to Flask (400 BAD REQUEST)

I'm currently learning Flask.

I've got multiple forms in my HTML and I want to send the form data to my backend in Flask. I've managed to successfully send over the data from a single form, but I'm having trouble with getting it to submit multiple forms at once.

Here's my JavaScript on getting the form data and appending each form:

function createUser() {
  var xhr = new XMLHttpRequest();
  var formData = new FormData()
  formData.append("userID", document.getElementById("newUserID").value);
  formData.append("userEmail", document.getElementById("newUserEmail").value);
  formData.append("userFirstName", document.getElementById("newUserFirstName").value);
  formData.append("userLastName", document.getElementById("newUserLastName").value);
  formData.append("userAvatar", document.getElementById("newUserAvatar").value);

  xhr.onreadystatechange = function() {
    if (xhr.readyState === 4 && xhr.status >= 200 && xhr.status < 300) {
      alert(xhr.responseText)
    }
  }
  xhr.open("POST", "api/add", true);
  xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded;charset=UTF-8');
  xhr.send(formData);
}

Here's my Flask:

@app.route('/api/add', methods = ['GET', 'POST'])
def post_user():
  if request.method == "POST":
    user = (request.form['UserID'],
            request.form['UserEmail'],
            request.form['UserFirstName'],
            request.form['UserLastName'],
            request.form['UserAvatar'])
  else:
    return render_template('users.html')
  user.append(user_list)

If I were to guess where I've gone wrong, it would be the formData.

Any help or pointers would be great. Thanks!