If you need to add custom field in cms pages then follow below steps:

1, You need to create a module for that so create module folders app/code/Magemonkey/CmspageBanner

2, Add registration.php file in your module Magemonkey/CmspageBanner

<?php 
/**
 * @author Magemonkey Team
 * @package Magemonkey_CmspageBanner
 */
MagentoFrameworkComponentComponentRegistrar::register(
 MagentoFrameworkComponentComponentRegistrar::MODULE,
 'Magemonkey_CmspageBanner',
 __DIR__
);

3, Add module.xml file in your module Magemonkey/CmspageBanner/etc

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
 <module name="Magemonkey_CmspageBanner" setup_version="1.0.1">
 </module>
</config>

4, Then create new schema file InstallSchema.php in  Magemonkey/CmspageBanner/setup

<?php
namespace MagemonkeyCmspageBannerSetup;

use MagentoFrameworkSetupInstallSchemaInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupSchemaSetupInterface;

class InstallSchema implements InstallSchemaInterface
{
 /**
 * Add Secondary Custom Content
 */
 public function install(SchemaSetupInterface $setup, ModuleContextInterface $context)
 {
 $installer = $setup;

 $installer->startSetup();

 $setup->getConnection()->addColumn(
 $setup->getTable('cms_page'),
 'banner_block',
 [
 'type' => MagentoFrameworkDBDdlTable::TYPE_TEXT,
 'length' => 255,
 'nullable' => true,
 'comment' => 'Banner Block Identifier'
 ]
 );

 $installer->endSetup();
 }
}

5, Then add your need to add a field in ui_component form file cms_page_form.xml in Magemonkey/CmspageBanner/vew/adminhtml/ui_component

<?xml version="1.0" encoding="UTF-8"?>
<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
 <fieldset name="general">
 <field name="banner_block">
 <argument name="data" xsi:type="array">
 <item name="config" xsi:type="array">
 <item name="dataType" xsi:type="string">text</item>
 <item name="label" xsi:type="string" translate="true">Banner Block Identifier</item>
 <item name="formElement" xsi:type="string">input</item>
 <item name="source" xsi:type="string">page</item>
 <item name="dataScope" xsi:type="string">banner_block</item>
 </item>
 </argument>
 </field>
 </fieldset>
</form>

6, Then you need to create one block file for the call that field in the frontend, Banner.php in Magemonkey/CmspageBanner/Block

<?php
namespace MagemonkeyCmspageBannerBlock;

use MagentoStoreModelScopeInterface;

class Banner extends MagentoFrameworkViewElementTemplate
{
 /**
 * @var MagentoCmsModelTemplateFilterProvider
 */
 protected $_filterProvider;

 /**
 * @var MagentoCmsModelPage
 */
 protected $_page;

 /**
 * Store manager
 *
 * @var MagentoStoreModelStoreManagerInterface
 */
 protected $_storeManager;

 /**
 * Page factory
 *
 * @var MagentoCmsModelPageFactory
 */
 protected $_pageFactory;

 /**
 * @var MagentoFrameworkViewPageConfig
 */
 protected $pageConfig;

 /**
 * Construct
 *
 * @param MagentoFrameworkViewElementTemplate $context
 * @param MagentoCmsModelPage $page
 * @param MagentoCmsModelTemplateFilterProvider $filterProvider
 * @param MagentoStoreModelStoreManagerInterface $storeManager
 * @param MagentoCmsModelPageFactory $pageFactory
 * @param MagentoFrameworkViewPageConfig $pageConfig
 * @param array $data
 */
 public function __construct(
 MagentoFrameworkViewElementTemplateContext $context,
 MagentoCmsModelPage $page,
 MagentoCmsModelTemplateFilterProvider $filterProvider,
 MagentoStoreModelStoreManagerInterface $storeManager,
 MagentoCmsModelPageFactory $pageFactory,
 MagentoFrameworkViewPageConfig $pageConfig,
 array $data = []
 ) {
 // used singleton (instead factory) because there exist dependencies on MagentoCmsHelperPage
 $this->_page = $page;
 $this->_filterProvider = $filterProvider;
 $this->_storeManager = $storeManager;
 $this->_pageFactory = $pageFactory;
 $this->pageConfig = $pageConfig;
 parent::__construct($context);
 }

 /**
 * Retrieve Page instance
 *
 * @return MagentoCmsModelPage
 */
 public function getPageData()
 {
 if (!$this->hasData('page')) {
 if ($this->getPageId()) {
 /** @var MagentoCmsModelPage $page */
 $page = $this->_pageFactory->create();
 $page->setStoreId($this->_storeManager->getStore()->getId())->load($this->getPageId(), 'identifier');
 } else {
 $page = $this->_page;
 }
 $this->setData('page', $page);
 }
 return $this->getData('page');
 }
}

After that you need to run setup:upgrade command

Then after you can call that block in your cms page Like:

<block class="MagemonkeyCmspageBannerBlockBanner" name="page.banner" template="Magemonkey_CmspageBanner::html/banner.phtml" after="-" />