Save image from canvas in Flask

Can anyone help me with saving canvas to my specific Flask folder? I retrieve image with canvas.toDataURL(), but have no idea how to upload it then

HTML:

{% block main %}
    <form action="/draw" method="post">
    <canvas id="canvas" style="border-style: solid;" width = "400" height = "400" ></canvas><hr>
    <button type="button" id="startsave">Save</button>

JS:

window.addEventListener('load', () => {
    document.getElementById('startsave').addEventListener('click', startsave);
});

function startsave() {
   var ref = canvas.toDataURL("image/png");
   var img = document.createElement("img");
   img.setAttribute("src", ref);
   img.setAttribute("name", "image");
   // TODO
}

Flask:

@app.route("/draw", methods=["GET", "POST"])
@login_required
def draw():
    if request.method == "GET":
        return render_template("/draw.html")
    else:
     #TODO