I am trying to add a second cms block in the category backend just below the default one. I copied and altered the category_form.xml and CategorySetup.php from core into a separate module.

I think I may be close as I can create the attribute and get a select element on the page. The issue is that the select box is empty - there are no options:

I am assuming I need to copy a source model or something like that but haven't found anything helpful online. Does anyone have some insight on this? I am running Magento EE 2.1.8.

Here is my relevant code:

InstallData.php

<?php
namespace [Vendor]\[Module]\Setup;

use Magento\Eav\Setup\EavSetup;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class InstallData implements InstallDataInterface
{
    private $eavSetupFactory;
    
    /**
     * Init
     *
     * @param EavSetupFactory $eavSetupFactory
     */
    public function __construct(EavSetupFactory $eavSetupFactory)
    {
        $this->eavSetupFactory = $eavSetupFactory;
    }

    /**
     * {@inheritdoc}
     * @SuppressWarnings(PHPMD.ExcessiveMethodLength)
     */

    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        /** @var EavSetup $eavSetup */
        $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);
        
        /**
         * Add attributes to the eav/attribute - landing_page_secondary
        **/
        
        $eavSetup->addAttribute(
            \Magento\Catalog\Model\Category::ENTITY, 
            'landing_page_secondary', [
                'type' => 'int',
                'label' => 'Secondary CMS Block',
                'input' => 'select',
                'source' => 'Magento\Catalog\Model\Category\Attribute\Source\Page',
                'required' => false,
                'sort_order' => 25,
                'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_STORE,
                'group' => 'Display Settings',
            ]
        );
                    
    }
}

category_form.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
/**
 * Copyright © 2013-2017 Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
    <fieldset name="content">
        <field name="landing_page_secondary">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="sortOrder" xsi:type="number">65</item>
                    <item name="dataType" xsi:type="string">string</item>
                    <item name="formElement" xsi:type="string">select</item>
                    <item name="label" xsi:type="string" translate="true">Add Secondary CMS Block</item>
                </item>
            </argument>
        </field>
    </fieldset>
</form>

Additionally, in Catalog/Block/Category/View.php I found this code for outputting the cms block. It does not reference the landing_page category attribute which is why another reason I think there are a few more classes I need to override/duplicate.

/**
 * @return mixed
 */
public function getCmsBlockHtml()
{
    if (!$this->getData('cms_block_html')) {
        $html = $this->getLayout()->createBlock(
            'Magento\Cms\Block\Block'
        )->setBlockId(
            $this->getCurrentCategory()->getLandingPage()
        )->toHtml();
        $this->setData('cms_block_html', $html);
    }
    return $this->getData('cms_block_html');
}