You should be able to draw with right mouse button. Is this how you try it? Does it work on other browsers?

I don't have a right mouse button. On OSX, I use ctrl+click as an equivalent (normally) to the right mouse button.

But it might be because of my convoluted setup (using the touchpad on Linux on a Macbook to control OSX on another Mac using barrier) so I'm not sure if it's actually relevant, don't sweat it.

Yeah, right-click drawing is not the most intuitive. I found it purely by accident.

Ah! In fact, it does. Thanks!