<div id="input_box">
  <input type="text" class="my_input_box_style" />
</div>

var dndSource = new dojo.dnd.AutoSource(dojo.byId("input_box"), {
  singular: true,
  copyOnly: true,
  selfCopy: false,
  selfAccept: false,
  accept: false
});

if(dndSource) {
  dojo.connect(dndSource, "onDrop", null, my_dnd_method, true);
}