Use the following code to insert the template block in WYSIWYG (Important! add it in inside double curly braces):
block class="Magento\Framework\View\Element\Template" template="Magefan_Blog::content/block-template-example.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.