From Magento 2.3.4, For the security enhancements, default custom layout update Textarea removed from the CMS page admin section.
If you want to add XML code snippet for the CMS Page, You must have to create a CMS Page layout handle for the page.
If you want to add Custom layout update code for the About Us page, You need to create a unique XML handle for the CMS page.
CMS Page Syntax:
cms_page_view_selectable_<PAGEIDENTIFIER>_<CUSTOMNAME>.xml
Example for the About Us page layout update,
PAGEIDENTIFIER indicates the identifier of the CMS Page(about-us)
CUSTOMNAME will be AboutPage, You can keep any desired name. (Use single word for the Custom name without space, dash or underscore)
Your actual handle will be,
cms_page_view_selectable_about-us_AboutPage.xml
Now, You have to create an XML file in theme or module level with code,
<?xml version="1.0"?>
<!--
/**
* Custom layout update handle CMS page Magento 2.3.4+.
*/
-->
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<!-- YOUR XML CODE -->
</body>
</page>
Now Go To Admin panel Login with credentials,
1. Click on Content -> Elements -> Pages
2. Select the CMS page, Click on Design tab,
3. Select Custom Layout Update from the dropdown as AboutPage.
4. Save Page from the Top right button.
5. Clear Cache.
Explore the storefront about the page to see your changes. You can do it for the Category Product Custom Layout Update.