I just did a screen-grab of the popup when you hover over the [+] to add a line, and it show [Ctrl] + [Alt] + ↓
However, in my system, this rotates my screen! Please select a keyboard shortcut that is not hooked to something else, or give users the ability to change it!
(NOTE: This is for Confluence Cloud... I already know the keyboard shortcut in Confluence Server [Alt]+↓ ... but that does NOT work for Confluence Cloud)