In order to add product widget to WYSIWYG editor you need to install Magento 2 Product Widget Advanced Extension. Once it's done you need to open any Admin panel page with a WYSIWYG editor. Then press the widget tool and choose Catalog Product Block

Magento 2 Product Widget Adding, Catalog Product Block

After that select, the product you want to display, set necessary options and press the Insert Widget button.

Magento 2 Product Widget Adding, Widget Options

Once you've inserted the widget, you'll see product block which you can check on the storefront. Don't forget to save changes by pressing the Save button.

Magento 2 Product Widget Adding