You cannot add shapes in Word Online.
Create a Freestyle Drawing
Cant find the shape you want?
Draw your own with the Freeform shapes.

You’ll find both of these shapes in the Line section of the Shapes list.
To change the color and appearance of a shape, select it, then selectFormat.
To switch to a different shape, selectFormat>Edit Shape>Change Shapeand choose the shape you want.

To create a drawing canvas:
To make your drawing canvas stand out, snag the canvas and selectFormat.
Give your canvas an outline shape or fill color.