You will need to extend the cms_page_form.xml
like
<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="custom_content">
<argument name="data" xsi:type="array">
<item name="config" xsi:type="array">
<item name="label" xsi:type="string"/>
<item name="formElement" xsi:type="string">wysiwyg</item>
<item name="source" xsi:type="string">page</item>
<item name="wysiwyg" xsi:type="boolean">true</item>
<item name="dataScope" xsi:type="string">custom_content</item>
<item name="additionalClasses" xsi:type="string">admin__field-wide</item>
</item>
</argument>
</field>
</fieldset>
</form>
This post will help you to achieve that.