Megamenu

Go to Appearance > Menus and click on Screen Options tab on top right of your screen. Under 'Show advanced menu properties' click checkbox on for 'CSS Classes'.

Determine the top level menu item (example: Elements) which serves as the main one.

Dropdown the menu item arrow and add the text megamenu in the metabox of 'CSS Classes (optional)'

Now, when you add 2nd level sub-menu item (example: Content) under the above one, you have to add the text heading in CSS Classes (optional).