Use the following code to insert the template block in WYSIWYG:

{{block class="Magento\Framework\View\Element\Template" template="VendorName_ModuleName::some-template.phtml" }}

The template identifier VendorName_ModuleName::some-template.phtml should be changed.

Attention! You can not use the block tag in the WYSIWYG product and category attributes. To make changes, install the magefan/module-catalog module.