There is no way to specifically detect the left mouse button click.
However, you can cover the entire map by trackables (or one biiig trackable), and detect its hit event. In that case, it will trigger for left and right mouse button alike, and there will be a few side effects like units' selection circles not appearing while being hovered. It will still not catch a mouse click inside the UI, like if the player clicks a unit's potrait for example.