// custom_widget.js
  var colorField = AbstractField.extend({
  });
  fieldRegistry.add("my_ztree", colorField);

// myView.xml
    <record id="library_book_view_form" model="ir.ui.view">
        <field name="name">Library Book Form</field>
        <field name="model">library.book</field>
        <field name="arch" type="xml">
            <form string="hr.department">
                <sheet>
                    <group>
                        <group>
                            <field name="name"/>
                            <field name="author_ids" widget="many2many_tags"/>
                        </group>
                        <group>
                            <field name="date_release"/>
                            <field name="treeData" widget="my_ztree" />
                        </group>
                    </group>
                </sheet>
            </form>
        </field>
    </record>