If you just want to open a new tab

To open a new tab in the current opened terminal you can press SHIFT+CTRL+T.
Alternatively, use the top level menu, which shows the keyboard shortcut (see screenshot below)

If you want to do it from the command line

Install xdotool - a program that lets you simulate keyboard input (among other things).

sudo apt-get install xdotool

then type in the terminal:

xdotool key ctrl+shift+t

That will simulate pressing the key combination, and open the new tab in the terminal.