I was thinking it was a vent at first since these include alkaline batteries. The Chinese version of the remotes uses 2032 cell batteries doesn't it?
Though I did look up a MiBox3 manual (non international I suppose) that shows a strap in the included list and showed instructions how to install it. The little hook inside is also inside the International MiBox3 remotes (at least mine does). But I noticed mine, new retail in box didn't come with a strap and the manual that shows included items didn't show a strap either.
So I guess a person could add a strap but if it's not 6 axis then no point.