create a custom panel-top row action

panel-top.js

({
    extendsFrom: 'PanelTopView',
  
    initialize: function(options) {
        this._super('initialize', [options]);
        
        var newEvents = {
        	'click a[name=custom_button]': 'customAction'
        }

        _.extend(this.events, newEvents);
    },
    
    customAction: function() {
        // do some custom stuff here
    },
})

panel-top.php

<?php

$viewdefs['SUBPANEL_MODULE_NAME']['base']['view']['panel-top'] = array(
    'buttons' => array(
        array(
            'type' => 'button',
            'css_class' => 'btn-invisible',
            'icon' => 'icon-chevron-up',
            'tooltip' => 'LBL_TOGGLE_VISIBILITY',
        ),
        array(
            'type' => 'actiondropdown',
            'name' => 'panel_dropdown',
            'css_class' => 'pull-right',
            'buttons' => array(
                array(
                    'type' => 'sticky-rowaction',
                    'icon' => 'icon-plus',
                    'name' => 'create_button',
                    'label' => ' ',
                    'acl_action' => 'create',
                    'tooltip' => 'LBL_CREATE_BUTTON_LABEL',
                ),
  // THIS IS THE CUSTOM ADDITION
                array(
                    'type' => 'rowaction',
                    'name' => 'custom_button',
                    'label' => ' Do Custom Stuff',
                    'css_class' => 'btn',
                    'icon' => 'icon-thumbs-up',
                ),
  // END CUSTOM ADDITION
            ),
        ),
    ),
);