Setting the Homepage URL in Craft 3
So it seems the way to do this is actually by leaving the slug blank. If you happen to look down on the section setup screen prior to naming your section you'll see it clearly: Leave blank if this is the homepage. However since that field is automatically filled in with details from the Name/Handle fields I never actually saw the text by the time I was down there unless I completely cleared the field out.

Both fields need to remain empty from what I can tell, not just the slug.