You can create a polygon by creating a table containing the vertices of your polygon.
Next, you can type "polygon" or use the poly command in the functions menu of the Desmos keyboard.
To see this example in action, click this link: https://www.desmos.com/calculator/4ha5mmpzdr
Another way to make a polygon is by using a list.
If you click and hold the icon on the polygon expression line, you'll see a list of style options.