Hi Hayden @Bonkerz ,
That very handy feature appears to have been removed from the BoolTool Addon. (since 2.80). If you really want to use it, you should use Blender 2.79 (or 2.78, which is used in the tutorial, if I’m not mistaken).
The paid Addon Boxcutter has that feature and (especially together with HardOps in a bundle), is definitely worth the money (comes with lifelong regular updates).
One relatively fast alternative way of doing this would be to Add a Single Vert (Addons > Add Mesh > Extra Objects), CTRL+LMB (if you are using LCS, that would be CTRL+RMB) to Extrude into a ‘Polygonal line’, select All and F to Fill. Then Add a Solidify Modifier to that Object and use it as a Boolean.