Ajax

html file

    <div class="input-group">
      <div class="input-group-prepend">
        <span class="input-group-text"><i class="fa fa-globe"></i></span>
      </div>
      <input placeholder="{{ session['domain'] }}" type="text" class="form-control" id="set_domain_input">
      <div class="input-group-append">
        <button class="btn btn-success" id="set_domain_btn">Set</button>
      </div>
    </div>

Preview 

You will need to set following 

<script type=text/javascript>
  $SCRIPT_ROOT = {{ request.script_root|tojson|safe }};
</script>

also

    <meta name="csrf-token" content="{{ csrf_token() }}">
    var csrftoken = $('meta[name=csrf-token]').attr('content');

    $.ajaxSetup({
      beforeSend: function(xhr, settings) {
        if (!/^(GET|HEAD|OPTIONS|TRACE)$/i.test(settings.type)) {
          xhr.setRequestHeader("X-CSRFToken", csrftoken)
        }
      }
    });

Alternatively, you can set

<meta name="csrf-token" content="{{ csrf_token() }}">
var csrf_token = "{{ csrf_token() }}";

$.ajax({
    type: 'POST',
    url: '/login',
    headers: {
        "X-CSRFToken": csrf_token,
    }
    data: {
     ...

Sample Ajax 

  $(document).ready(function() {

    $(document).on('click', '#set_domain_btn', function() {
          var csrf_token = "{{ csrf_token() }}";
          var domain = $('#set_domain_input').val();
          data = {"domain": domain};

          $.ajax({
              url: "{{ url_for('toor.setdomain') }}",
              contentType: 'application/json;charset=UTF-8',
              data: JSON.stringify(data, null, '\t'),
              headers: {                  "X-CSRFToken": csrf_token,              },
              type: 'POST',
              dataType: 'json',
              success: function(response) {
                  console.log(response);
                  window.location.reload();
              },
              error: function(error) {
                  console.log(error);
              }
          });
      });

  });

python 

@app.route('/set', methods=['GET', 'POST'])
def setdomain():
    if request.method == "POST":
        r = request.get_json()
        # r = request.get_json(force=True)
        domain = r['domain']
        print(domain)
        return jsonify({"domain": domain})

How to manually add csrf token in html form

<form id="regForm" action="/login" method="POST" role="post">
          <input id="csrf_token" name="csrf_token" type="hidden" value="{{ csrf_token }}">
          <input type="submit" value="Submit" />
</form>