RequestStart

<span id="target" title="Tooltip content">
  Some content
</span>

<script>
  $(document).ready(function() {
    $("#target").kendoTooltip({
      iframe: false,
      content: {
        url: "https://demos.telerik.com/kendo-ui/content/web/tooltip/ajax/ajaxContent3.html"
      },
      width: 220,
      height: 280,
      requestStart: function(e) {
/* The result can be observed in the DevTools(F12) console of the browser. */
        console.log("request is started");
      }
    });
  });
</script>

<span id="target" title="Tooltip content">
  Some content
</span>

<script>
  $(document).ready(function() {
    var tooltip = $("#target").kendoTooltip({
      iframe: false,
      content: {
        url: "https://demos.telerik.com/kendo-ui/content/web/tooltip/ajax/ajaxContent3.html"
      },
      width: 220,
      height: 280
    }).data("kendoTooltip");

    tooltip.bind("requestStart", function(e) {
/* The result can be observed in the DevTools(F12) console of the browser. */
      console.log("request is started");
    });
  });
</script>