MacRumors published a review on the Belkin Key Ring and Strap
A quote from the article:
Apple designed the AirTags without any kind of hole or attachment point to connect them to keys, bags, and other items, which has opened up a whole new market for AirTag accessories.Belkin Key Ring and Strap Review
Apple's own AirTag add-ons are priced starting at 29, which is the same price as a standalone AirTag, so it's quite an investment to use one of those. Luckily there are plenty of third-party AirTag accessories that are already available on the market, and Belkin is one of the first companies to come out with an AirTags solution.