<script>

    data = {'ID': 'foobar'};

    $.ajax({
        type: "POST",
        url: "/test/ajax",
        contentType: 'application/json;charset=UTF-8',
        data: JSON.stringify(data, null, '\t'),
        dataType: 'json',
        success: function(data) {
            window.location.href = data['url']
        },
        error: function(response) {
            alert(response)
        },
    });

</script>  

@app.route('/test/ajax', methods=['GET', 'POST'])
def test_ajax():

    if request.method == "POST":
        data = request.json
        print('data:', data)

        result = {'url': url_for('index')}
        print('result:', result)

        return jsonify(result)

    else:
        return render_template('test-ajax.html')