ColumnMenuInit

<div id="treeList"></div>
<script>
  $("#treeList").kendoTreeList({
    columns: [
      { field: "name" },
      { field: "age" }
    ],
    dataSource: {
      data: [
        { id: 1, parentId: null, name: "Jane Doe", age: 22 },
        { id: 2, parentId: 1, name: "John Doe", age: 24 }
      ]
    },
    columnMenu: true,
    columnMenuInit: function(e) {
      var menu = e.container.find(".k-menu").data("kendoMenu");
      var field = e.field;
      menu.append({ text: "Custom" });
      menu.bind("select", function(e) {
        if ($(e.item).text() == "Custom") {
/* The result can be observed in the DevTools(F12) console of the browser. */
          console.log("Custom button for", field);
        }
      });
    }
  });
</script>

<div id="treeList"></div>
<script>
  function treelist_columnMenuInit(e) {
    var menu = e.container.find(".k-menu").data("kendoMenu");
    var field = e.field;
    menu.append({ text: "Custom" });
    menu.bind("select", function(e) {
      if ($(e.item).text() == "Custom") {
/* The result can be observed in the DevTools(F12) console of the browser. */
        console.log("Custom button for", field);
      }
    });
  }

  $("#treeList").kendoTreeList({
    columns: [
      { field: "name" },
      { field: "age" }
    ],
    dataSource: {
      data: [
        { id: 1, parentId: null, name: "Jane Doe", age: 22 },
        { id: 2, parentId: 1, name: "John Doe", age: 24 }
      ]
    },
    columnMenu: true
  });

  var treelist = $("#treeList").data("kendoTreeList");
  treelist.bind("columnMenuInit", treelist_columnMenuInit);
</script>