create a custom subpanel-list row action

subpanel-list.js

// this file also goes in custom/SUBPANEL_MODULE_NAME/clients/base/views/subpanel-list/

({
    extendsFrom: 'SubpanelListView',

    initialize: function(options) {
        this._super('initialize', [options]);
        
        this.context.on('button:custom_action:click', this.customAction, this);
    },

    customAction: function() {
        // this is where you put your custom code
    },
})

subpanel-list.php

<?php

// this file goes in: custom/SUBPANEL_MODULE_NAME/clients/base/views/subpanel-list/
// so for example if you want to update the contacts subpanel that appears on Accounts
// you would sub Contacts for SUBPANEL_MODULE_NAME

$viewdefs['SUBPANEL_MODULE_NAME']['base']['view']['subpanel-list'] = array(
    'template' => 'recordlist',
    'favorite' => true,
    'rowactions' => array(
        'actions' => array(
            array(
                'type' => 'rowaction',
                'name' => 'edit_button',
                'icon' => 'icon-pencil',
                'label' => 'LBL_EDIT_BUTTON',
                'event' => 'list:editrow:fire',
                'acl_action' => 'edit',
                'allow_bwc' => true,
            ),
/*~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  THIS IS THE CUSTOM PART
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~*/
            array(
                'type' => 'rowaction',
                'css_class' => 'btn',
                'event' => 'button:custom_action:click',
                'icon' => 'icon-bolt', // check out style guide for more options: /#Styleguide/docs/base-icons
                'label' => ' Custom SHIZ',
                'acl_action' => 'view',
                'allow_bwc' => false,
// end custom addition                
            ),
        ),
    ),
    'last_state' => array(
        'id' => 'subpanel-list',
    ),
);