<div draggable="true" id="textbox" >

<div ondragover="allowDrop(event)"   >
    <div id="canvas"   class="grid container">
        <div   style="color:red"  data-dojo-attach-point="canvasNode"></div>
    </div>
</div>

return declare([_WidgetBase, _TemplatedMixin, _WidgetsInTemplateMixin], {

    templateString: template,

    allowDrop: function (ev) {
        alert("test");
        ev.preventDefault();
    },
    ....
  }

<script>
    function allowDrop(ev) {
        ev.preventDefault();
    }

    function drag(ev) {
        ev.dataTransfer.setData("text", ev.target.id);
    }

    function drop(ev) {
        ev.preventDefault();
        var data = ev.dataTransfer.getData("text");
        //ev.target.appendChild(document.getElementById(data));
    }
</script>

<div data-dojo-attach-event= "ondragover:allowDrop(event)">
    <div id="canvas"   class="grid container">
        <div style="color:red"  data-dojo-attach-point="canvasNode"></div>
    </div>
</div>