[roll.]1 d 20[/roll.]
There's a button for it too. You just type the number you want to roll, the letter "D", and the number of sides on the die.
EDIT: I had to redo it because even in code it still rolled for some reason... So that's why there are spaces in it.
EDIT2: Redid it again... With .'s inside this time.
Here's the finished product:
Rolled 1d20 : 19, total 19