Hello,
In the Woodmart demos, the mega menu uses extra menu list widgets, so you’ll need to edit the corresponding HTML block and manually add your subcategories there.
If you prefer to have subcategories added automatically instead of manually, you can create a regular WordPress menu with your categories/subcategories and then use the Menu widget inside the HTML block to display them dynamically.
Please refer articles below for creating mega menus: https://xtemos.com/docs-topic/create-mega-menu/, https://surli.cc/rqbbth
Best Regards,