Here’s how you can create a drop-down by adding a child menu to a parent menu item:
Go to Website builder > Navigation
Create the top level menu item, if not already created, using “Add New External Link” button on top. See screenshot.
Then, click on the icon (see screenshot) besides the menu item which you want to display in the drop-down and select the parent menu item.
Click on Save.
Do it for all other menu items which you want to display in the drop-down.