So after watching the tutorial, it doesnt seem like any scripting is necessary. Basically you need two scripts from the unity github account for navmesh runtime. Add those to your game objects and select the area to cover.
I havent tried it, but I suspect you could also make an empty gameobject, add the script, set the size, make it a prefab. Then you can use playmaker to place the prefab at run time, covering the new area you created. (Which will then be added to the navmesh runtime updater).
Checkout the tutorial around 4 minutes I believe.