How to save a file automatically when clicking on a button

I am developing an application in html, php and some javascript, surely someone has already read my doubts here. I have a canvas where I make a signature for the users and I have in my form, a button in which I convert the canvas into a png image and the windows box automatically appears to select the folder to download, but I don't want that. I want the signature to be automatically downloaded to a folder. Attached html code and JavaScript

Canvas definition:

<canvas id="canvas"> 
        Su navegador no soporta canvas
</canvas>

This is the code snippet for the transform button:

<script type="text/javascript">
    function guardar() {        
      var link = document.createElement('a')
      link.download = "firma";

      link.href = canvas.toDataURL("image/png").replace("image/png", "image/octet-stream");
      link.click();
    }
</script>

Canvas events:

<script type="text/javascript">
    var limpiar = document.getElementById("limpiar");
    var canvas = document.getElementById("canvas");
    var ctx = canvas.getContext("2d");
    var cw = canvas.width = 150, cx = cw / 2;
    var ch = canvas.height = 150, cy = ch / 2;

    var dibujar = false;
    var factorDeAlisamiento = 5;
    var trazados = [];
    var puntos = [];
    ctx.lineJoin = "round";


    limpiar.addEventListener('click', function(evt){
        dibujar = false;
        ctx.clearRect(0, 0, cw, ch);
        trazados.length = 0;
        puntos.length = 0;
    }, false);


    canvas.addEventListener('mousedown', function(evt) {
         dibujar = true;
         //ctx.clearRect(0, 0, cw, ch);
         ctx.beginPath();
    }, false);

    canvas.addEventListener('mouseup', function(evt) {
        dibujar = false;
    }, false);

    canvas.addEventListener("mouseout", function(evt) {
          dibujar = false;
    }, false);

    canvas.addEventListener("mousemove", function(evt) {
        if (dibujar) {
            var m = oMousePos(canvas, evt);
            puntos.push(m);
            ctx.lineTo(m.x, m.y);
            ctx.stroke();
          }
    }, false);

    function reducirArray(n,elArray) {
        var nuevoArray = [];
         nuevoArray[0] = elArray[0];
         for (var i = 0; i < elArray.length; i++) {
            if (i % n == 0) {
              nuevoArray[nuevoArray.length] = elArray[i];
            }
          }
        nuevoArray[nuevoArray.length - 1] = elArray[elArray.length - 1];
        Trazados.push(nuevoArray);
    }

    function calcularPuntoDeControl(ry, a, b) {
        var pc = {}
        pc.x = (ry[a].x + ry[b].x) / 2;
        pc.y = (ry[a].y + ry[b].y) / 2;
        return pc;
    }

    function alisarTrazado(ry) {
        if (ry.length > 1) {
        var ultimoPunto = ry.length - 1;
        ctx.beginPath();
        ctx.moveTo(ry[0].x, ry[0].y);
        for (i = 1; i < ry.length - 2; i++) {
            var pc = calcularPuntoDeControl(ry, i, i + 1);
            ctx.quadraticCurveTo(ry[i].x, ry[i].y, pc.x, pc.y);
        }
        ctx.quadraticCurveTo(ry[ultimoPunto - 1].x, ry[ultimoPunto - 1].y, ry[ultimoPunto].x, ry[ultimoPunto].y);
        ctx.stroke();
          }
    }

    function redibujarTrazados(){
        dibujar = false;
        ctx.clearRect(0, 0, cw, ch);
        reducirArray(factorDeAlisamiento,puntos);
        for(var i = 0; i < Trazados.length; i++){
          alisarTrazado(Trazados[i]);
        }
    }

    function oMousePos(canvas, evt){
        var ClientRect = canvas.getBoundingClientRect();
        return { //objeto
            x: Math.round(evt.clientX - ClientRect.left),
            y: Math.round(evt.clientY - ClientRect.top)
          }
    }       
</script>

I would be missing the code or part of the code, to know how to save the image in a specific path, a specific folder.

A greeting and thanks in advance.